telemetry: session_close covers the whole close

[davidzhao]

What

session_close now spans the whole of AgentSession._aclose_locked: the activity teardown, the close event's handlers, the session host and room io. agent_session ends, and its root context is cleared, only after all of that.

Why

The close span ended after the activity teardown, then agent_session ended, and the close event, session host and room io closed afterwards while the session's context was still current in the closing task. Anything emitted there parented to an already ended span. In a real export, a 103 ms event_loop_blocked from the ai_coustics enhancer's native free during room_io.aclose() showed up 8 ms after agent_session had closed, poking out of it.

With this change that stall nests inside session_close, where it happened, and a close handler's own spans do too.

Tests

test_session_lifecycle_spans_and_events registers a close handler that opens a span and asserts it nests under session_close, that session_close ends after it and agent_session after that, and that the root context is cleared.

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