fix: Update FileDataSource to process reload triggers under lock.

[kinyoklion]

Put the load under lock so only a single load is happening concurrently.

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Note

Synchronizes LoadAll() in FileDataSource with a lock to prevent concurrent reloads from file watcher events.

• Server SDK – File data loading:
  • Add _updateLock and wrap LoadAll() body in lock to serialize reloads and avoid concurrent loads from file watcher.
  • All versioning, parsing, and _dataSourceUpdates.Init(...) now occur within the critical section in pkgs/sdk/server/src/Internal/DataSources/FileDataSource.cs.

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