Add SwitchReason enum to categorize provider switches:
- RateLimit: 429 errors
- ServiceIssue: 5xx errors
- CircuitBreaker: circuit breaker open

Each reason maps to a generic user-facing message:
- RateLimit -> "high demand"
- ServiceIssue -> "a temporary service issue"
- CircuitBreaker -> "service maintenance"

Includes comprehensive unit tests for enum functionality and cloning.

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Add configuration struct for provider switch notifications with:
- enabled flag (defaults to true via serde)
- default_message field with professional default template
- Default trait implementation
- Serde serialization/deserialization support

Tests verify:
- Default configuration contains IMPORTANT prefix
- Serialization roundtrip works
- enabled defaults to true when not specified

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Add public re-exports for ProviderSwitchNotificationConfig and SwitchReason
to make notification types accessible to consumers of lunaroute-routing.

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- Add provider_switch_notification field to RoutingConfig struct
- Field is optional and defaults to None
- Add tests for serialization with and without notification config
- Tests verify YAML deserialization works correctly

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- Add switch_notification_message field to OpenAIConfig
- Add switch_notification_message field to AnthropicConfig
- Update constructors to initialize field with None default
- Add tests for both configs to verify field works
- Update all test constructors to include new field
- Update server provider initialization with None default
- Update integration test constructors with None default

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Adds substitute_template_variables function that replaces template variables
in notification messages with actual values:
- ${original_provider}: Provider that failed
- ${new_provider}: Provider being switched to
- ${reason}: Generic reason for switch
- ${model}: Model name from request

Includes 5 comprehensive tests covering all variables, no variables, partial
variables, duplicates, and special characters.

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Adds build_notification_message function that constructs notification messages
by combining template selection and variable substitution:
- Uses provider-specific override message if available
- Falls back to global default message
- Substitutes all template variables (provider names, reason, model)

Includes 3 tests covering default message, provider override, and variable
substitution.

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Adds has_notification_already function that prevents duplicate notifications
during cascading failovers by checking if the first message already starts
with "IMPORTANT:".

Includes 4 comprehensive tests:
- Notification present (returns true)
- Notification absent (returns false)
- Empty messages array (returns false)
- Multimodal message (returns false, doesn't check non-text content)

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…cations

Complete Phase 4 tasks from provider-switch-notification implementation plan:

Task 4.1: Add notification config to Router struct
- Added notification_config field to Router struct
- Updated Router::new() to accept notification_config parameter
- Updated Router::with_defaults() to pass None for notification_config
- Updated all Router::new() test calls with None parameter

Task 4.2: Implement provider config lookup
- Added get_provider_notification_message() method to Router
- Method retrieves custom notification message from provider configs
- Initially stubbed, then completed in Task 4.3

Task 4.3: Extend Provider trait
- Added get_notification_message() method to Provider trait with default impl
- Implemented for OpenAIConnector returning switch_notification_message
- Implemented for AnthropicConnector returning switch_notification_message
- Updated Router's get_provider_notification_message() to use trait method

Task 4.4: Implement inject notification logic
- Added inject_notification_if_needed() method to Router
- Checks if notifications are enabled via config
- Uses has_notification_already() for idempotency guard
- Builds notification using build_notification_message()
- Prepends notification message to request messages array
- Returns true if notification was injected, false otherwise

All tests passing:
- lunaroute-routing: 147 tests passed
- lunaroute-core: 44 tests passed
- lunaroute-egress: 17 tests passed
- lunaroute_integration_tests: 27 tests passed

Note: Methods are currently unused (marked with #[allow(dead_code)])
because Phase 5 integration into error handling flow is not yet
implemented. This is intentional and per the implementation plan.

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This commit integrates the notification injection system into the actual
provider switching flow at three critical points:

1. **Rate limit switching (LimitsAlternative strategy)**: When a primary
   provider returns a rate limit error and the router switches to an
   alternative provider, a notification is injected into the request
   explaining the switch due to "high demand".

2. **Fallback switching (5xx errors, circuit breaker, other errors)**:
   When the primary provider fails and the router tries fallback providers,
   a notification is injected based on the error type:
   - 5xx errors trigger "service issue" notifications
   - Circuit breaker open triggers "service maintenance" notifications
   - Other errors default to "service issue" notifications

3. **Streaming fallback (circuit breaker)**: When streaming requests are
   routed to fallback providers due to circuit breaker state, a notification
   is injected with the "service maintenance" reason.

Implementation details:
- Added `is_5xx_error()` helper method to detect server errors by checking
  for 5xx status codes and error messages
- Removed `#[allow(dead_code)]` attributes from notification methods
- Notification injection occurs by cloning the request, prepending the
  notification message, and passing the modified request to the alternative
  or fallback provider
- Error type flags (`is_rate_limit_error`, `is_5xx_error`) are declared
  before the match and assigned within the error arm to ensure they remain
  in scope for the fallback logic

All existing tests continue to pass, demonstrating that the integration
maintains backward compatibility with current routing behavior.

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Issue 1: Simplified circuit breaker reason logic
- Removed misleading circuit breaker check in fallback switch_reason
- Circuit breaker check was always false after trying provider
- Now uses ServiceIssue for all non-rate-limit errors
- Removed unused is_5xx_error function as distinction no longer needed

Issue 2: Eliminated need for 5xx detection
- Both 5xx and non-5xx errors now use ServiceIssue switch reason
- Simplified logic to only distinguish rate limits from other errors
- Removed is_5xx_error variable and function entirely

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- Test rate limit switch injects notification
- Test notification can be disabled
- Test cross-dialect notification (OpenAI → Anthropic)
- Test provider-specific custom message overrides
- Test idempotency guard prevents duplicate notifications

All 5 tests passing

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- Demonstrates global notification configuration
- Shows provider-specific custom messages with template variables
- Includes GPT models with OpenAI→Anthropic rate limit protection
- Includes Claude models with Anthropic→OpenAI fallback
- Documents all notification feature capabilities

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- Added section in Advanced Configuration
- Documented features, configuration, and template variables
- Included reference to example config file
- Shows both global and per-provider customization options

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Changes:
- Move metrics initialization before router creation in main.rs
- Change Router::with_defaults() to Router::new() to accept notification config
- Pass config.routing.provider_switch_notification to Router
- Add log message when provider switch notifications are enabled

This completes Phase 7 Task 1 - server now properly passes notification
configuration from YAML/database config through to the routing layer.

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