Preserve Guardian parent response IDs across sampling requests (#45441)

## Why

Running code-mode cells can request Guardian review while the next response is in flight. Clearing the response ID before that response emits `response.created` leaves those reviews without a `parent_response_id`.

## What changed

Keep the latest response ID received in the turn until a later `response.created` replaces it, including across sampling retries.

## Testing

Add regression coverage for reviews before and after a response handoff and for a fresh turn that must not inherit the previous turn's ID. Update retry coverage to expect the last known parent when the retry supplies no response ID.

GitOrigin-RevId: af0a08de09edad59e1a7ade7904a42d616b05679

codex-rs/codex-api/src/common.rs

@@ -72,7 +72,7 @@ pub struct MemorySummarizeOutput {
     pub memory_summary: String,
 }
 
-/// The server response currently being handled, shared with tool-review extensions.
+/// The latest server response ID received in this turn, shared with tool-review extensions.
 #[derive(Clone, Debug)]
 pub struct ResponseId(pub String);