Register agent tasks behind use_agent_identity (#17387)

## Summary

Stack PR3 for feature-gated agent identity support.

This PR adds per-thread agent task registration behind
`features.use_agent_identity`. Tasks are minted on the first real user
turn and cached in thread runtime state for later turns.

## Stack

- PR1: https://github.com/openai/codex/pull/17385 - add
`features.use_agent_identity`
- PR2: https://github.com/openai/codex/pull/17386 - register agent
identities when enabled
- PR3: https://github.com/openai/codex/pull/17387 - this PR, original
task registration slice
- PR3.1: https://github.com/openai/codex/pull/17978 - persist and
prewarm registered tasks per thread
- PR4: https://github.com/openai/codex/pull/17980 - use `AgentAssertion`
downstream when enabled

## Validation

Covered as part of the local stack validation pass:

- `just fmt`
- `cargo test -p codex-core --lib agent_identity`
- `cargo test -p codex-core --lib agent_assertion`
- `cargo test -p codex-core --lib websocket_agent_task`
- `cargo test -p codex-api api_bridge`
- `cargo build -p codex-cli --bin codex`

## Notes

The full local app-server E2E path is still being debugged after PR
creation. The current branch stack is directionally ready for review
while that follow-up continues.

codex-rs/app-server/tests/suite/v2/plugin_list.rs

@@ -30,7 +30,8 @@ use wiremock::matchers::method;
 use wiremock::matchers::path;
 use wiremock::matchers::query_param;
 
-const DEFAULT_TIMEOUT: Duration = Duration::from_secs(10);
+// These tests start full app-server processes and can also run plugin startup warmers.
+const DEFAULT_TIMEOUT: Duration = Duration::from_secs(60);
 const TEST_CURATED_PLUGIN_SHA: &str = "0123456789abcdef0123456789abcdef01234567";
 const STARTUP_REMOTE_PLUGIN_SYNC_MARKER_FILE: &str = ".tmp/app-server-remote-plugin-sync-v1";