Make extension lifecycle hooks async

[jif-oai]

Why

Extension lifecycle hooks sit on the host/extension boundary, but the current trait surface only allows synchronous callbacks. That forces extensions that need to seed, rehydrate, observe, or flush extension-owned state during thread and turn transitions to either block inside the callback or move async work into separate host plumbing.

This PR makes those lifecycle callbacks awaitable so extension implementations can perform async work directly at the lifecycle point where the host already has the relevant session, thread, or turn stores available.

What changed

• Makes ThreadLifecycleContributor and TurnLifecycleContributor async in codex-extension-api.
• Awaits thread start/resume/stop and turn start/stop/abort lifecycle callbacks from codex-core.
• Updates the guardian and memories extensions to implement the async lifecycle trait surface.
• Updates the existing lifecycle tests to use async contributor implementations.
• Adds async-trait to the crates that now expose or implement these async object-safe lifecycle traits.

Testing

• Existing codex-core lifecycle tests were updated to cover async implementations for thread stop and turn abort ordering.

[jif-oai]

@codex review

[chatgpt-codex-connector]

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