Fix transaction wrapper mutex release on exception

[Solution-lab]

Summary

Fixes a mutex leak when a FileFsClient operation throws before EndTransaction is called.

Changes

• Replaced manual BeginTransaction / EndTransaction calls with callback-based Execute.
• Ensured the mutex is always released after an operation.
• Updated client tests to use NullTransactionWrapper.
• Added a regression test for mutex release after an exception.

Known limitation

FileFsClient.Copy was already implemented without a transaction and remains unchanged in this PR.

Verification

• 497 tests passed
• Full solution build completed with 0 errors