Surface Flex capacity failures as a distinct terminal error (#47967)

## What changed

- Recognize `flex_unavailable` in HTTP 429 responses and streamed `error` and `response.failed` events. End normal turns without retries and report `Flex capacity unavailable.`
- Expose `flexUnavailable` through the core and app-server protocols and generated schemas and SDK types.
- Keep Flex capacity failures eligible for Guardian review and sampler retries, with a distinct telemetry classification.
- Map unexpected HTTP responses to `HttpConnectionFailed`, preserving their status codes.

## Testing

Add regression coverage for terminal Flex failures over HTTP, SSE, and WebSocket, HTTP 504 status preservation after a stream retry, and protocol serialization. Update Guardian retry and sampler recovery tests to cover Flex failures.

GitOrigin-RevId: f2e03a6d2d36a6d9748c35952e40397f8670d121

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -63,6 +63,7 @@
             "sessionBudgetExceeded",
             "usageLimitExceeded",
             "rateLimitExceeded",
+            "flexUnavailable",
             "serverOverloaded",
             "cyberPolicy",
             "misalignmentPolicyViolation",