## Why
`slow_down` errors were treated as terminal server overloads, while exhausted credit balances and spend limits fell through to retryable stream errors.
## What changed
- Classify `slow_down` as a retryable rate limit in HTTP 503 responses and SSE failures, preserving the server message and parsing retry delays from SSE error messages.
- Map `credit_balance_exhausted`, `organization_spend_limit_exceeded`, and `project_spend_limit_exceeded` SSE errors to quota exhaustion so they terminate without retries.
## Testing
Extend tests to cover HTTP error classification and retryability, `slow_down` stream retry exhaustion and message-provided delays, and a single `UsageLimitExceeded` error with no retries for each quota error code.
GitOrigin-RevId: 7985eba71ba4a6fe897a45957e448fadd42f3963
## What changed
Add the default-off `api_key_model_discovery` feature and expose it through app-server experimental feature enablement, respecting user configuration overrides.
When enabled, fetch OpenAI API-key model metadata from the Codex backend, honoring explicit provider base URLs. Treat remote catalogs containing visible models as authoritative. When disabled at startup, skip discovery requests and cached catalogs and retain bundled models.
## Testing
Add coverage for feature enablement and user overrides, cached catalogs across sessions, provider routing, and API-key `model/list` results. Extend coverage for remote instructions and auth-specific cache isolation.
GitOrigin-RevId: 4b189c37c615a7f808cb6ff5f0858a9a55f36ebc
## Why
The overload integration test coupled terminal error behavior to live retry timing and tracing events.
## What changed
- Verify retry attempts and exponential backoff directly with `run_with_retry` under paused Tokio time.
- Keep the overload integration test focused on request count and terminal error events.
- Add timeouts while waiting for retry telemetry, resumed requests, and turn completion so failures cannot hang the test.
GitOrigin-RevId: 571a6a6a36a6aded038000a63bff1958c7209913
## What changed
- Classify `response.failed` events with the `rate_limit_exceeded` code as a distinct retryable error while preserving any parsed retry delay.
- Expose the error as `rateLimitExceeded` through the core protocol and app-server schemas after stream retries are exhausted.
- Preserve the upstream message for TUI display while keeping it out of telemetry summaries.
## Testing
- Cover SSE classification, retry metadata, protocol conversion and serialization, exhausted stream retries, telemetry redaction, and TUI rendering.
GitOrigin-RevId: 02dab4d3477dcd7653a58c49c4bd38687a616579
Steve Coffey