Work around metadata-only PDB emit failures

[jaredpar]

Metadata-only compilations can retain PDB-specific emit options that current Roslyn mishandles, causing EmitToMemory failures or invalid CodeView metadata. Roslyn PR dotnet/roslyn#85028 fixes this upstream, but that fix is not yet available in a package complog can reference.

This temporarily normalizes metadata-only EmitOptions while CompilerLogReader rehydrates a compilation:

• clears the PDB path because metadata-only output has no associated PDB
• replaces embedded debug information with portable PDB format to avoid the current Roslyn guard
• keeps the workaround local to reader option deserialization so it remains transparent to callers

Adds regression coverage demonstrating the upstream Roslyn failure and links the related issue and fix.

Testing

• dotnet test src\Basic.CompilerLog.UnitTests\Basic.CompilerLog.UnitTests.csproj --framework net10.0 --no-restore --blame-hang --blame-hang-timeout 60s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (4c99388) to head (b76bb6d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   95.83%   95.85%   +0.01%     
==========================================
  Files          54       54              
  Lines        6174     6175       +1     
  Branches      710      710              
==========================================
+ Hits         5917     5919       +2     
  Misses        138      138              
+ Partials      119      118       -1     

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