[RTDB] Fix concurrency crash in FView

[paulb777]

Fix #15514

Also tried to restore the disabled flaky integration tests in FData.m and FIRDatabaseQueryTests.m. The FData.m ones still flake but the one in FIRDatabaseQueryTests.m has passed multiple runs, so leaving enabled.

The quickstart CI failure seems unrelated - probably related to ongoing spm transition.

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[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a concurrency crash in FView by introducing synchronization around accesses to the eventRegistrations mutable array. The use of @synchronized is appropriate and correctly applied to all read and write operations on the array, effectively preventing race conditions. The fix is solid. I've added one minor suggestion to modernize the code for removing event registrations, which could improve readability and maintainability.