Support timeout and cancellation for session turns

[overlord-letta]

Summary

runTurn() does not appear to support a per-turn timeout or AbortSignal. Consumers that need bounded runtime behavior have to wrap runTurn()/send() in Promise.race() and then close the session manually.

Proposed surface

Support timeout/cancellation at the turn level, for example:

await session.runTurn(message, { timeoutMs: 120_000 })
await session.runTurn(message, { signal })

The same cancellation story should ideally apply to send() plus stream() flows.

Why this matters

Consumers often need to distinguish:

• a turn that is still running,
• a turn that timed out locally,
• a transport/request timeout,
• a runtime failure,
• a user-initiated cancellation.

External Promise.race() wrappers lose that structure and can leave the SDK/session doing work unless the caller remembers to close or abort the session.

Acceptance criteria

• runTurn() supports either timeoutMs, AbortSignal, or both.
• Timeout/cancellation produces a typed or at least documented terminal outcome.
• The SDK cleans up stream listeners/session state after timeout/cancellation.
• Examples show the recommended pattern for long-running cloud turns.