This commit refactors the service section index mapping code in ServiceTab.swift to improve readability and efficiency.

The changes:
- Simplified the enumerated loop by adding a `where` clause directly to the iteration
- Reduced nesting by moving the sectionServices.contains check into the loop condition
- Improved code clarity by eliminating the unnecessary if block inside the loop
- Net result: 5 insertions, 7 deletions (more concise and readable code)

This refactoring makes the code more maintainable and easier to understand while preserving the exact same functionality for mapping service indices between sections and the full service array.

…ect needPrivateAPIKey usage

This commit refactors the API key requirement logic across multiple services to use the direct `needPrivateAPIKey()` method, eliminating complex conditional checks.

Key changes:
- Removed the `requiresAPIKey(_:)` helper method from ServiceTab (29 lines deleted)
- Updated all service implementations to override `needPrivateAPIKey()` directly:
  - Built-in services (Apple, Bing, DeepL, Google, etc.) return `false`
  - OpenAI StreamService returns dynamic value based on `requireAPIKey`
  - Base QueryService returns `true` as default
- Simplified service filtering in ServiceTab to use `$0.needPrivateAPIKey()` directly

This refactoring:
- Reduces code duplication and complexity
- Makes API key requirements explicit in each service
- Improves maintainability by centralizing the logic
- Eliminates complex runtime type checking and string matching

…nto unified apiKeyRequirement

This commit consolidates the API key requirement system by introducing a unified `apiKeyRequirement()` method that replaces both `requireAPIKey` and `needPrivateAPIKey`.

Key improvements:
- Introduces `ServiceAPIKeyRequirement` enum with three cases: `.none`, `.builtIn`, `.userProvided`
- Replaces 15+ `needPrivateAPIKey()` implementations with `apiKeyRequirement()` returning appropriate enum values
- Removes `requireAPIKey` property from StreamService (it was always `true`)
- Updates all service implementations to use the new enum system:
  - Free services (Apple, Bing, DeepL, Google, etc.) return `.none`
  - Built-in services (AITool, BuiltInAI) return `.builtIn`
  - User API services (OpenAI, DeepL Pro, etc.) return `.userProvided`
- Updates ServiceTab filtering logic to use `apiKeyRequirement().needsUserProvidedKey`
- Updates validation logic to check `apiKeyRequirement().requiresKeyForRequest`

This refactoring provides a more expressive and type-safe way to categorize API key requirements across all services.