OpenAI can return streamed overload errors with the codes
`server_is_overloaded` and `slow_down`. Zed previously only recognized
`overloaded_error`, so these responses fell through to
`ProviderErrorCategory::Other`. Downstream consumers could then treat a
routine provider-capacity failure as an unexpected error.

Classify all three codes as `ProviderErrorCategory::Overloaded`. This
lets existing retry and error-reporting behavior handle them
consistently without matching provider-specific messages outside the
OpenAI response parser.

This matches Codex’s handling. Codex classifies both
[`server_is_overloaded` and `slow_down` in streamed
responses](https://github.com/openai/codex/blob/a26f1806a4f4b8cfec2ea1be129963815a61e58c/codex-rs/codex-api/src/sse/responses.rs#L696-L699),
and performs the [same classification for HTTP 503 response
bodies](https://github.com/openai/codex/blob/a26f1806a4f4b8cfec2ea1be129963815a61e58c/codex-rs/codex-api/src/api_bridge.rs#L63-L74).

Release Notes:

- Improved OpenAI error handling