RUM-6171: Use FileObserver

[kikoveiga]

What does this PR do?

This is the follow-up on #2720, which was rolled back in #3124.
The fixes can be found in this commit, which include:

• Add existsSafe call inside getReadableFile(), so that we don't return a stale file that no longer exists on disk.
• Improve thread safety, by using synchronized and grouping the state into currentBatchState: AtomicReference<CurrentBatchState>.
• Instead of using direct equality between event and FILE_OBSERVER_MASK, use bitmask logic to guarantee matches when some additional bits might have been set.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.38554% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.02%. Comparing base (39d8b4a) to head (9f7d1cb).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
...al/persistence/file/batch/BatchFileOrchestrator.kt	96.39%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3168      +/-   ##
===========================================
+ Coverage    70.96%   71.02%   +0.06%     
===========================================
  Files          912      912              
  Lines        33548    33577      +29     
  Branches      5640     5648       +8     
===========================================
+ Hits         23804    23845      +41     
+ Misses        8173     8162      -11     
+ Partials      1571     1570       -1     

Files with missing lines	Coverage Δ
...al/persistence/file/batch/BatchFileOrchestrator.kt	94.35% <96.39%> (+1.45%)	⬆️

... and 35 files with indirect coverage changes

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[aleksandr-gringauz]

Between currentBatchState.set() and knownFiles.add(), there's a brief window where:

1. currentBatchState points to the new file
2. knownFiles doesn't contain it yet

If listBatchFiles() is called during this window, it won't include the new file, causing getReusableWritableFile() to potentially return null when it shouldn't.