Route response item IDs through a shared generator (#59312)

## What changed

Introduce `ItemIdGenerator` and pass the runtime-owned generator through sessions, model clients, history, hooks, tool outputs, and memory requests. Keep UUIDv7 generation as the default and replace direct calls to `ResponseItemId::new` with the shared callable.

Preserve existing item IDs and backfill missing or empty IDs through the same generator, including forked history before reconstruction and persistence.

## Testing

Extend coverage for UUIDv7 IDs on user messages, agent messages, and additional tools, and assert that forked model context matches the backfilled items persisted to history.

GitOrigin-RevId: 0dd9ccf0f46f0f1005a6acfc740133b03ce82259

codex-rs/core/src/codex_thread.rs

@@ -342,6 +342,11 @@ impl CodexThread {
         &self.session.services.thread_extension_data
     }
 
+    /// Generator used for response items produced on behalf of this thread.
+    pub fn item_id_generator(&self) -> &codex_protocol::ItemIdGenerator {
+        &self.session.item_id_generator
+    }
+
     /// Returns tools from this thread’s ready, unshared Apps MCP client.
     /// Returns an empty list if the client is absent, still starting, or uses
     /// a shared catalog. Does not start or wait for discovery.