Transfer - Snapshot may be incomplete

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• All static analysis checks passed.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.

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This pull request guarantees that when adding a directory to the repository snapshot, it will happen in real-time, not deferred as a separate task. This is crucial for the following reason:

Imagine the following directory structure:

Root -> file1
Root -> dir1

Before this modification, we might upload the file and then mark "Root" as "DONE" because we've uploaded all files in Root, and the snapshot manager believes there are no children of "Root."
With this change, it ensures that "dir1" will be included in the snapshot before file1 is uploaded. This guarantees that "Root" won't be designated as completed until "dir1" is also completed.

This pull request introduces two more changes:

1. It eliminates unnecessary snapshot manager locking in WasSnapshotLoaded to enhance efficiency.
2. It now generates an error when an attempt is made to decrease a node's filesCount when filesCount is already at 0.

[github-actions]

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