[RC] Fix data race on gIsNewDatabase

[paulb777]

Fix #14715

Gemini's analysis:
✦ I've analyzed RCNConfigDBManager.m and confirmed gIsNewDatabase is accessed in RemoteConfigCreateFilePathIfNotExist and
isNewDatabase without proper synchronization. While dispatch_sync is present, the gIsNewDatabaseQueue is initialized too late
within sharedInstance. To fix this, I'll move the queue's initialization into a +load method, ensuring it's set up before any
other class methods are called. I will now apply this change.

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[paulb777]

/gamini review

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request effectively resolves a potential data race on the gIsNewDatabase global variable. Moving the initialization of gIsNewDatabaseQueue to the +load method is a robust approach in Objective-C, guaranteeing that the queue is set up before any other methods in the class are called, thus preventing the race condition. The addition of a new unit test, testIsNewDatabaseThreadSafety, is excellent as it specifically verifies the thread safety of the fix by simulating concurrent access. The changes are correct, well-targeted, and improve the overall stability of the RCNConfigDBManager.