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/models-manager/src/model_info_tests.rs

@@ -82,6 +82,7 @@ fn base_instruction_override_is_literal_and_preserves_catalog_messages() {
         computer_use: Some("  # Native policy\r\n\n${native_markdown}\n".to_string()),
     };
     let mut messages = ModelMessages {
+        content_filter_guidance: None,
         persistent_instructions: Some(persistent_instructions.to_string()),
         tools: Some(ToolMessages {
             send_user_message_async: Some(ToolMessage {