feat: add turn lifecycle contributors

[jif-oai]

Why

Extensions can already contribute prompt, tool, turn-item, and thread-lifecycle behavior, but there was no explicit host-owned hook for per-turn setup and cleanup. That makes extension-private turn state awkward: an extension either has to stash it outside the turn lifecycle or depend on core runtime objects.

This adds a small turn lifecycle boundary. Extensions receive stable identifiers plus the existing session, thread, and turn ExtensionData stores, while core keeps owning task scheduling, cancellation, and turn teardown.

What Changed

• Added TurnLifecycleContributor with on_turn_start, on_turn_stop, and on_turn_abort callbacks in codex-rs/ext/extension-api.
• Added typed TurnStartInput, TurnStopInput, and TurnAbortInput payloads that expose thread_id, turn_id, session_store, thread_store, and turn_store.
• Registered and re-exported turn lifecycle contributors through ExtensionRegistry and ExtensionRegistryBuilder.
• Wired Session to emit turn start, stop, and abort callbacks from the existing turn/task lifecycle paths.
• Carried the turn-scoped ExtensionData through RunningTask and RemovedTask so stop/abort callbacks receive the same turn store created at turn start.

Verification

• Not run locally.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

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