Preserve VFS immutable-write metadata

[KSemenenko]

VFS immutable writes accept caller metadata, but a subsequent file metadata read drops all raw fields. The legacy prefix filter also matches its own namespace and hides user fields. This blocks publication of stored Learning materials which must verify their owner and digest.

Return raw portable metadata and legacy prefixed user fields while excluding reserved VFS bookkeeping fields. Preserve cached/fresh reads, immutable retries and ETags. Increment the canonical patch version to 10.0.15 and document the corrected contract.

Validation:

• Four real Azure/VFS cases reproduced missing metadata before the fix, covering ASCII/Unicode names with caching on/off.
• Focused provider/VFS suite: 121 passed.
• Required non-BrowserStress suite: 503 passed, 3 existing skips, 506 total.
• Canonical solution formatting and final Release solution build passed with zero warnings or errors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.70%. Comparing base (78e93b4) to head (454e446).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   50.33%   50.70%   +0.37%     
==========================================
  Files         164      164              
  Lines        8112     8111       -1     
  Branches     1082     1082              
==========================================
+ Hits         4083     4113      +30     
+ Misses       3560     3528      -32     
- Partials      469      470       +1     

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