[API] Fix issue with Baggage.Current leaking

[martincostello]

Changes

Fix issue with Baggage.Current updates made in a child async flow that could leak back into the parent flow.

I got Copilot to write most of this PR. It seems reasonable to me, but this probably needs quite a critical review.

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 89.80%. Comparing base (ed44995) to head (252796b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7191      +/-   ##
==========================================
- Coverage   89.86%   89.80%   -0.07%     
==========================================
  Files         275      275              
  Lines       13852    13856       +4     
==========================================
- Hits        12448    12443       -5     
- Misses       1404     1413       +9     

Flag	Coverage Δ
unittests-Project-Experimental	89.61% <100.00%> (-0.16%)	⬇️
unittests-Project-Stable	89.79% <100.00%> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
src/OpenTelemetry.Api/Baggage.cs	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

[Copilot]

Pull request overview

This PR fixes an ambient-context bug where Baggage.Current mutations performed in child async/parallel flows could leak back into the parent flow by removing shared mutable state from the runtime context slot.

Changes:

• Update Baggage to store an immutable BaggageHolder in the runtime context slot and replace the holder on every update (instead of mutating a shared holder instance).
• Add/adjust unit tests to validate isolation of Baggage.Current across async and parallel flows.
• Add a changelog entry describing the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
test/OpenTelemetry.Api.Tests/BaggageTests.cs	Adds/updates tests for async/parallel flow isolation of Baggage.Current.
src/OpenTelemetry.Api/CHANGELOG.md	Documents the baggage leak fix in Unreleased notes.
src/OpenTelemetry.Api/Baggage.cs	Implements the leak fix by replacing the context slot value on updates and making the holder immutable.

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[Kielek]

There were some previously reverted changes to this scope.

See the issue #3257
Previous fix #5208 with the comment requesting revert #5208 (comment)

Revert itself #5227

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@martinjt, @danelson, @CodeBlanch, FYI