…ity-error fallback for explicit providers

Closes #26803

Root causes:
1. _is_payment_error() checked for billing keywords (credits, insufficient
   funds, billing, payment required) but missed daily token quota exhaustion
   phrases used by Bedrock, Vertex AI, and LiteLLM proxies — e.g.
   'Too many tokens per day', 'quota exceeded', 'resource exhausted',
   'daily limit'. These are functionally identical to credit exhaustion
   (provider cannot serve the request) but don't trigger fallback.

2. The call_llm() fallback chain was gated on resolved_provider == 'auto'.
   When a task resolves to a specific provider (e.g. 'custom' for a LiteLLM
   proxy, or 'openrouter'), capacity failures (payment/quota/connection)
   silently raise instead of trying alternatives. This is overly conservative:
   capacity errors mean the provider *cannot* serve the request regardless of
   user intent, so alternatives should always be tried.

Fixes:
- Add quota-related keywords to _is_payment_error(): quota_exceeded,
  too many tokens per day, daily limit, tokens per day, daily quota,
  resource exhausted (Vertex AI gRPC code).
- Allow fallback for capacity errors (payment + connection) even when
  resolved_provider is not 'auto'. Rate-limit fallback stays gated on
  is_auto to honour explicit provider constraints for transient limits.
- Apply both fixes to sync call_llm() and async acall_llm() paths.
- Add 6 targeted tests for the new quota-error detection cases.

… net

Layered fallback for auxiliary tasks (compression, vision, tts, web_extract,
session_search, etc.):

  1. Primary aux provider (existing)
  2. User-configured auxiliary.<task>.fallback_chain (new)
  3. Main agent provider + model (new — last-resort safety net)
  4. Warn user + re-raise original error (new)

For users on 'auto' (no explicit aux provider), the existing
_try_payment_fallback auto-detection chain runs instead — its Step 1
already IS the main agent model, so they get the same behaviour without
configuration.

The configured fallback_chain config schema comes from #26882 / @zccyman;
the main-agent safety net + exhaustion warning were added on top.

Closes #26882. Builds on the capacity-error gate fix in the previous
commit (#26803 / @Bartok9).

Adds a new 'Auxiliary Capacity-Error Fallback' section to
website/docs/user-guide/features/fallback-providers.md covering:

- The 4-step ladder (primary → fallback_chain → main agent → warn)
- Which errors trigger fallback (402, 429 quota, connection) vs
  which respect explicit provider choice (transient 429 rate limits)
- Optional fallback_chain config schema with vision + compression examples
- Recognized quota-error phrases (Bedrock, Vertex AI, generic)

Updates the bottom summary table — every auxiliary task now shows
'Layered (see above)' instead of 'Auto-detection chain' since
explicit-provider users also get the main-agent safety net.