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.rs

@@ -94,6 +94,11 @@ pub enum CodexErrorDetails {
     /// The Session loop treats this as a transient error and will automatically retry the turn.
     #[error("stream disconnected before completion: {0}")]
     Stream(String),
+    /// A response stopped by the content filter. Sampling retries need developer guidance.
+    #[error(
+        "stream disconnected before completion: Incomplete response returned, reason: content_filter"
+    )]
+    ContentFilter,
     /// A retryable upstream rate limit received inside the response stream.
     #[error("rate limit exceeded: {0}")]
     RateLimitExceeded(String),
@@ -411,6 +416,7 @@ impl CodexErr {
             | CodexErrorDetails::BioPolicy { .. }
             | CodexErrorDetails::MisalignmentPolicyViolation { .. } => None,
             CodexErrorDetails::Stream(..)
+            | CodexErrorDetails::ContentFilter
             | CodexErrorDetails::RateLimitExceeded(_)
             | CodexErrorDetails::Timeout
             | CodexErrorDetails::RequestTimeout