Keep rollout trace terminal sessions distinct when process IDs are reused (#56011)

## Why

Runtime process handles are recyclable and thread-local. Using them as terminal session identities can merge separate commands or associate `write_stdin` operations with the wrong exec.

## What changed

- Give each exec a distinct terminal session and track the latest session for each thread-local process handle.
- Record the exec selected by `write_stdin` at runtime and use it to correct session membership during replay, including when dispatch is delayed until a handle is reused.
- Fall back to the latest session for the thread and handle when replaying older traces without runtime target events.

## Testing

Extend reducer coverage for process ID reuse across threads and successive execs, stdin session membership, and delayed runtime target resolution.

GitOrigin-RevId: 330c6fd26a4eebdef03ae600a030dc76920e0dd9

codex-rs/core/src/unified_exec/process_manager.rs

@@ -996,6 +996,15 @@ impl UnifiedExecProcessManager {
         } = self
             .prepare_process_handles(process_id, &locked_process)
             .await?;
+        context
+            .session
+            .services
+            .rollout_thread_trace
+            .record_stdin_target(
+                &context.step_context.turn.sub_id,
+                &context.call_id,
+                &call_id,
+            );
         let remote_exited =
             process.has_exited() && output.drain_policy == OutputDrainPolicy::WaitForOutputClosure;
         // A write racing with remote exit still collects the final output.