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/protocol/src/error_tests.rs

@@ -40,6 +40,7 @@ async fn codex_err_debug_preserves_legacy_shape() {
 fn retryability_preserves_error_details_distinctions() {
     let errors = [
         (CodexErr::ServerOverloaded, false),
+        (CodexErr::new(CodexErrorDetails::ContentFilter), true),
         (
             CodexErr::new(CodexErrorDetails::RateLimitExceeded("retry later".into())),
             true,