## Why
#24744 introduced the thread idle lifecycle hook so idle continuation
can be owned by lifecycle contributors instead of hard-coded goal
runtime plumbing. Task completion still called
`goal_runtime_apply(GoalRuntimeEvent::MaybeContinueIfIdle)` directly, so
the post-turn idle transition remained goal-specific and did not notify
generic thread lifecycle contributors.
## What Changed
- Add `Session::emit_thread_idle_lifecycle_if_idle()` to gate idle
emission on both no active turn and no queued trigger-turn mailbox work.
- Call that helper when a task clears the active turn, replacing the
direct `GoalRuntimeEvent::MaybeContinueIfIdle` path.
- Cover the behavior with `codex-core` session tests for emitting after
task completion and suppressing idle emission while trigger-turn mailbox
work is pending.
## Verification
- New tests in `core/src/session/tests.rs` exercise the idle lifecycle
emission and trigger-turn mailbox guard.
462deb0426