chore: remove redundant mutex lock around log4cxx warning

[achintya2k5]

Description:
Remove redundant mutex lock around log4cxx warning in the network update loop. The underlying log4cxx::Logger is internally thread-safe, making the outer mMutex lock unnecessary and a potential source of lock contention.

• Remove std::unique_lock from the catch block in Client::scheduleNetworkUpdate
• Preserve the concurrency structure to ensure the deadlock fix from PR fix: resolve network update thread deadlock by releasing mutex before… #1533 remains fully intact

Related issue(s):

Fixes #1611

Notes for reviewer:
Built locally using the linux-x64-debug CMake preset.
Ran the full ClientUnitTests suite (24/24 passed). Specifically verified concurrency safety by running the NetworkUpdateThreadDoesNotDeadlock* suite; all 6 tests passed in < 1 second with 0 timeouts, confirming no regression of #1533.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

Hey @achintya2k5 👋 thanks for the PR!
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✅ Issue Link -- Linked to #1611 (assigned to you).

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