fix(sessions): resolve async deadlock in multiplexed session manager

[sinhasubham]

This PR resolves a critical deadlock issue when acquiring or maintaining a multiplexed session asynchronously.
The bug occurs because DatabaseSessionsManager previously used a synchronous threading.Lock around self._get_multiplexed_session() and _maintain_multiplexed_session(). When a thread attempts to await the multiplexed session creation (return await ...) while holding a synchronous thread lock, the entire asyncio event loop becomes blocked for any other coroutine trying to access the lock.

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical deadlock that previously occurred in the asynchronous multiplexed session manager. The core problem stemmed from using a synchronous threading.Lock in an asynchronous context, which caused the entire asyncio event loop to block when a coroutine attempted to await while holding the lock. By migrating to an asynchronous CrossSync.Lock, the system can now correctly manage concurrent access to shared resources without freezing the event loop, thereby enhancing the stability and performance of asynchronous operations.

Highlights

• Deadlock Resolution: Resolved a critical deadlock issue in the asynchronous multiplexed session manager by replacing threading.Lock with CrossSync.Lock. This change allows async with statements to be used with the lock, preventing the asyncio event loop from blocking when awaiting session creation or maintenance.
• Concurrency Safety: Ensured that concurrent acquisition and maintenance of multiplexed sessions can proceed without blocking, significantly improving the reliability and responsiveness of the Spanner client library in asynchronous contexts.
• Code Style Refinements: Applied consistent tuple unpacking syntax across various files, simplifying assignments and improving code readability.
• New Unit Test: Added a dedicated unit test to verify that concurrent calls to _get_multiplexed_session no longer result in a deadlock, confirming the effectiveness of the fix.

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[gemini-code-assist]

Code Review

This pull request effectively resolves a critical deadlock in the asynchronous multiplexed session manager by correctly switching from a synchronous threading.Lock to an async-aware CrossSync.Lock. The inclusion of a new concurrency test is a great addition that validates the fix. Most other changes are stylistic improvements. I've identified one minor issue involving what appears to be dead code, which should be removed for better code clarity.

[bhatt4982]

LGTM...