feat(runtime-fallback): add timeout toggle for quota retry detection

Make provider auto-retry signal detection respect timeout_seconds setting:
- When timeout_seconds=0, disable quota-based fallback escalation
- Only treat auto-retry signals as errors when timeout is enabled
- Add test to verify behavior when timeout_seconds is disabled
- Update documentation to explain timeout_seconds=0 behavior

This allows users to disable timeout-based fallbacks while keeping
error-based fallback functionality intact.

docs/configurations.md

@@ -738,7 +738,7 @@ Automatically switch to backup models when the primary model encounters retryabl
 | `retry_on_errors`       | `[429, 503, 529]` | HTTP status codes that trigger fallback (rate limit, service unavailable). Also supports certain classified provider errors (for example, missing API key) that do not expose HTTP status codes.   |
 | `max_fallback_attempts` | `3`               | Maximum fallback attempts per session (1-10)                                |
 | `cooldown_seconds`      | `60`              | Cooldown in seconds before retrying a failed model                          |
-| `timeout_seconds`       | `30`              | Timeout in seconds for an in-flight fallback request before forcing the next fallback model                          |
+| `timeout_seconds`       | `30`              | Timeout in seconds for an in-flight fallback request before forcing the next fallback model. Set to `0` to disable timeout-based fallback and provider quota retry signal detection. |
 | `notify_on_fallback`    | `true`            | Show toast notification when switching to a fallback model                  |
 
 ### How It Works
@@ -924,7 +924,7 @@ Automatically switch to backup models when the primary model encounters retryabl
 | `retry_on_errors`       | `[429, 503, 529]` | HTTP status codes that trigger fallback (rate limit, service unavailable). Also supports certain classified provider errors (for example, missing API key) that do not expose HTTP status codes.   |
 | `max_fallback_attempts` | `3`               | Maximum fallback attempts per session (1-10)                                |
 | `cooldown_seconds`      | `60`              | Cooldown in seconds before retrying a failed model                          |
-| `timeout_seconds`       | `30`              | Timeout in seconds for an in-flight fallback request before forcing the next fallback model                          |
+| `timeout_seconds`       | `30`              | Timeout in seconds for an in-flight fallback request before forcing the next fallback model. Set to `0` to disable timeout-based fallback and provider quota retry signal detection. |
 | `notify_on_fallback`    | `true`            | Show toast notification when switching to a fallback model                  |
 
 ### How It Works