[WCF] Add test for .NET Framework

[martincostello]

See #4026 (comment)

Changes

Add test missed from #4026.

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 75.99%. Comparing base (3d15483) to head (3cc9ecd).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4318      +/-   ##
==========================================
- Coverage   75.99%   75.99%   -0.01%     
==========================================
  Files         466      466              
  Lines       18577    18577              
==========================================
- Hits        14118    14117       -1     
- Misses       4459     4460       +1     

Flag	Coverage Δ
unittests-Instrumentation.Wcf	81.44% <ø> (-0.14%)	⬇️

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see 1 file with indirect coverage changes

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[Copilot]

Pull request overview

Adds a missing regression test in the WCF instrumentation test suite to cover the .NET Framework service-side dispatch path when an operation’s Action is null (related to the fix from #4026).

Changes:

• Add a NETFRAMEWORK-only unit test that constructs an EndpointDispatcher with a DispatchOperation whose Action is null.
• Assert the dispatch behavior path does not throw and registers a message inspector.

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