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/codex-api/src/api_bridge_tests.rs

@@ -64,7 +64,13 @@ fn map_api_error_distinguishes_capacity_from_slow_down() {
             false,
         ),
         ("slow_down", CodexErrorInfo::RateLimitExceeded, true),
-        ("unknown_error", CodexErrorInfo::Other, true),
+        (
+            "unknown_error",
+            CodexErrorInfo::HttpConnectionFailed {
+                http_status_code: Some(503),
+            },
+            true,
+        ),
     ] {
         let err = map_api_error(ApiError::Transport(TransportError::Http {
             retry_after: None,