Minor bugfixes from static analysis

[waf]

Fix a few issues uncovered from static analysis (missing using on IDisposable, null issues, multithreading access on non-thread-safe collections).

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.5%. Comparing base (f7dadfc) to head (23ce9c8).

Files with missing lines	Patch %	Lines
...s/SymbolExploration/SourceLink/SourceLinkLookup.cs	50.0%	2 Missing and 1 partial ⚠️
CSharpRepl.Services/Configuration.cs	0.0%	1 Missing ⚠️
...ces/Roslyn/References/DotNetInstallationLocator.cs	87.5%	0 Missing and 1 partial ⚠️
CSharpRepl.Services/Theming/StyledString.cs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #506     +/-   ##
=======================================
- Coverage   80.5%   80.5%   -0.1%     
=======================================
  Files        130     130             
  Lines       9462    9481     +19     
  Branches    1417    1427     +10     
=======================================
+ Hits        7618    7633     +15     
- Misses      1354    1356      +2     
- Partials     490     492      +2     

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