Support metadata staging for reserved thread IDs (#38819)

## Why

Callers need to associate host-owned state with a thread before Core starts it.

## What changed

- Add `ThreadManager::reserve_thread_id` and allow new threads to start with the reserved ID. Reject reserved IDs when resuming an existing thread.
- Let thread stores stage metadata for a reserved ID. The local store merges staged values into the first successful metadata update and then removes them.
- Clear staged metadata when an unmaterialized thread shuts down or is discarded, and require the state database for staging.

## Testing

- Cover reserved ID generation and resume validation.
- Cover metadata persistence, merging, rollout-compatible updates, cleanup, and invalid staging requests.

GitOrigin-RevId: 3f9bc48f0f7764011cc90ea01349e497dda0a9e3

codex-rs/core/tests/common/Cargo.toml

@@ -27,6 +27,7 @@ codex-login = { workspace = true }
 codex-model-provider-info = { workspace = true }
 codex-models-manager = { workspace = true }
 codex-protocol = { workspace = true }
+codex-thread-store = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 codex-utils-path-uri = { workspace = true }