Add recovery guidance to content-filter retries (#49119)

## Why

Responses stopped by `content_filter` previously followed generic stream-error retry handling without guidance about the block. Retried requests need instructions to explain the limitation and offer a permitted alternative.

## What changed

- Recognize content-filter failures as a distinct error while preserving retryability and the existing public error representation.
- Append developer guidance to the conversation after each sampling block, retaining delivered output and using the existing retry handling.
- Support a model catalog override through `model_messages.content_filter_guidance`. Missing, null, blank, or values exceeding 512 UTF-8 bytes fall back to bundled guidance.
- The bundled guidance tells the model to avoid reproducing or working around blocked content, briefly explain the limitation, offer a permitted alternative, and continue unrelated authorized work.

## Testing

Add a scenario and request-history snapshot covering repeated blocks, catalog guidance on each retry, retained partial output, and eventual completion. Add error-mapping assertions for retryability, public error classification, and error text.

GitOrigin-RevId: ba0c201b3a5a3acf5cfa971332027240e0690225

codex-rs/cli/src/doctor.rs

@@ -2429,7 +2429,8 @@ fn websocket_error_detail(err: &ApiError) -> String {
             format!("handshake API error: {status} {message}")
         }
         ApiError::Stream(message) => format!("handshake stream error: {message}"),
-        ApiError::ContextWindowExceeded
+        ApiError::ContentFilter
+        | ApiError::ContextWindowExceeded
         | ApiError::QuotaExceeded
         | ApiError::UsageNotIncluded
         | ApiError::Retryable { .. }