Harden directory storage

[bsneed]

• Add file validation before handing off to uploader.
• Check if files are finalized before appending to them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.18%. Comparing base (6e9ec95) to head (f809dfc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   70.42%   71.18%   +0.75%     
==========================================
  Files          49       49              
  Lines        3686     3699      +13     
==========================================
+ Hits         2596     2633      +37     
+ Misses       1090     1066      -24     

Files with missing lines	Coverage Δ
...gment/Utilities/Storage/Types/DirectoryStore.swift	85.36% <100.00%> (+1.72%)	⬆️

... and 2 files with indirect coverage changes

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[lalmalani]

PR Addresses 1 optimization to prevent a hard-to-repro race condition.
We check for file completion via a newly introduced function (BOOL) isFInished (which checks to see if there is a "sentAt" string match in the last line of file.
If isFinished returns true, we check again and start a new file with an incremented filename