[Extensions.Hosting.Tests] Nullable

[ysolomchenko]

Towards #3958

Changes

Extensions.Hosting.Tests mark as nullable

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.33%. Comparing base (6250307) to head (d9c0c26).
Report is 336 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5862      +/-   ##
==========================================
+ Coverage   83.38%   86.33%   +2.95%     
==========================================
  Files         297      257      -40     
  Lines       12531    11214    -1317     
==========================================
- Hits        10449     9682     -767     
+ Misses       2082     1532     -550     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.00% <ø> (?)
unittests-Project-Stable	86.21% <ø> (?)

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see 234 files with indirect coverage changes

[TimothyMothra]

Did you intend to do two projects in this PR; OpenTelemetry.Extensions.Hosting.Tests and OpenTelemetry.Tests?

[ysolomchenko]

@TimothyMothra some files from the OpenTelemetry.Tests project were imported using include, so I made them nullable. However, I would prefer to update OpenTelemetry.Tests in a separate PR.