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/realtime_conversation.rs

@@ -72,6 +72,7 @@ use wiremock::matchers::path;
 use wiremock::matchers::path_regex;
 
 const DEFAULT_TIMEOUT: Duration = Duration::from_secs(10);
+const DELEGATED_SHELL_TOOL_TIMEOUT_MS: u64 = 30_000;
 const STARTUP_CONTEXT_HEADER: &str = "Startup context from Codex.";
 const V2_STEERING_ACKNOWLEDGEMENT: &str =
     "This was sent to steer the previous background agent task.";
@@ -1717,7 +1718,9 @@ async fn webrtc_v2_tool_call_delegated_turn_can_execute_shell_tool() -> Result<(
         create_shell_command_sse_response(
             realtime_tool_ok_command(),
             /*workdir*/ None,
-            Some(5000),
+            // Windows CI can spend several seconds starting the nested PowerShell command. This
+            // test verifies delegated shell-tool plumbing, not timeout enforcement.
+            Some(DELEGATED_SHELL_TOOL_TIMEOUT_MS),
             "shell_call",
         )?,
         create_final_assistant_message_sse_response("shell tool finished")?,