Surface environment startup failure reasons to the model (#44277)

## Why

Failed environments were omitted from environment context, and `wait_for_environment` returned a generic failure or treated an already-failed environment as unselected. The model needs the failure reason to explain problems such as an empty repository.

## What changed

- Preserve the selected environment and error when startup or configuration fails.
- Include failed status and XML-escaped error details in environment context, and return the reason from `wait_for_environment` for failures detected before or during a wait.
- Limit error details to 256 bytes per environment and 512 bytes total in context, preserving UTF-8 boundaries. Limit wait failure details to 256 bytes.

## Testing

Add coverage for failures before a turn and while waiting, verifying that the model receives the reason and the turn completes. Add unit tests for XML escaping, incremental context updates, clearing errors on recovery, and UTF-8-safe truncation.

GitOrigin-RevId: 093ab9b447c7b3243fbb53b665180db95479f2a7

codex-rs/core/tests/suite/remote_env.rs

@@ -4023,3 +4023,6 @@ async fn remote_test_env_copy_preserves_symlink_source() -> Result<()> {
         .await?;
     Ok(())
 }
+
+#[path = "remote_env_failure_tests.rs"]
+mod failure_tests;