[OTLP] refactor: trust custom CA only

[sandy2008]

Fixes #6800
Design discussion issue #6800

Changes

• Enforce custom CA pinning even when system trust succeeds by always building the chain against the provided CA.
• Add a regression test that ensures a mismatched CA is rejected even if SslPolicyErrors.None.

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)

[sandy2008]

@rajkumar-rangaraj Please review! #6804

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.85%. Comparing base (866e218) to head (fc6bba5).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...yProtocol/Implementation/OtlpCertificateManager.cs	84.61%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6804      +/-   ##
==========================================
- Coverage   86.88%   86.85%   -0.03%     
==========================================
  Files         262      262              
  Lines       12350    12355       +5     
==========================================
+ Hits        10730    10731       +1     
- Misses       1620     1624       +4     

Flag	Coverage Δ
unittests-Project-Experimental	86.76% <84.61%> (-0.11%)	⬇️
unittests-Project-Stable	86.64% <84.61%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...yProtocol/Implementation/OtlpCertificateManager.cs	73.91% <84.61%> (-2.03%)	⬇️

... and 2 files with indirect coverage changes

[rajkumar-rangaraj]

LGTM

[github-actions]

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