Fall back to HTTP immediately for oversized WebSocket requests (#60840)

## Why

A WebSocket close with code `1009` before any text arrives indicates that the request exceeds the peer's message size limit. Retrying the request over another WebSocket does not address that limit.

## What changed

- Classify these early size rejections as requiring HTTP fallback and switch transports without exhausting WebSocket retries, including during prewarming.
- Preserve ordinary stream retry behavior for size closes after any text frame and for other close codes.

## Testing

Add integration coverage for early size rejections during generation and prewarming, size closes after metadata, rate limits, or response creation, and ordinary closes.

GitOrigin-RevId: 4550d289bb414fec1faae2a195bcc635d394117b

codex-rs/codex-api/src/error.rs

@@ -15,6 +15,8 @@ pub enum ApiError {
     Api { status: StatusCode, message: String },
     #[error("stream error: {0}")]
     Stream(String),
+    #[error("websocket request exceeds the peer's message size limit (close code 1009)")]
+    WebsocketMessageTooLarge,
     #[error("stream error: Incomplete response returned, reason: content_filter")]
     ContentFilter,
     #[error(transparent)]