[Manohar Paturi (ManoharPaturi)]

Motivation & Context

TodoProvider serialized its read-modify-write todo operations with a lock that belonged to the provider instance. Todo stores are deliberately shareable, so two providers over one store serialized against nothing: both could load the same state before either saved, reuse the same item id, and the last save silently dropped the other write while both tool calls reported success. The lock was also keyed by session only, so a session reused across asyncio.run boundaries could raise from the loop bound lock, the exact hazard the file access layer documents and guards against.

Description & Review Guide

• What are the major changes? The lock registry moved from the provider to a module level map keyed by session, then by store identity and event loop, so every provider over one store and session shares one lock, mirroring _store_write_lock in the file access layer.
• What is the impact of these changes? Concurrent todo mutations from different agents sharing a store now serialize exactly like mutations from one agent, and per loop keys keep a cross asyncio.run session working. The outer map stays weak, so locks for garbage collected sessions are still evicted, and the default in memory store keeps its zero await point fast path.
• What do you want reviewers to focus on? The regression gate placed after tool capture, because before_run itself loads todo state to inject it into the run, and the eviction test now asserting against the module registry.

Related Issue

Fixes #9868

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[Manohar Paturi (ManoharPaturi)]

Understood, and sorry. #9868 now follows the bug report template with the race reproduction, and this PR stays closed until the team agrees with the locking direction.