Align Baggage API name and value handling to match the v1.36.0 specification

[zacharycmontoya]

Partially addresses #5210
Design discussion issue: N/A

Changes

This PR updates the OpenTelemetry.Baggage type so that it handles baggage name and values in accordance with the v1.36.0 specification:

• Baggage names and values are treated as case sensitive
• Baggage names are any valid, non-empty UTF-8 strings
• Baggage values are any valid UTF-8 strings

No changes were required to handle UTF-8 strings, but new tests were added to assert this behavior.

⚠️ Although the API contract has not changed, this is a BREAKING change as users of this API who previously inserted baggage items whose names only differ in case-sensitivity will now see individual baggage items, whereas they would previously collide.

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 88.51%. Comparing base (bb25768) to head (fbc4fd0).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6931      +/-   ##
==========================================
+ Coverage   88.49%   88.51%   +0.01%     
==========================================
  Files         263      263              
  Lines       12408    12411       +3     
==========================================
+ Hits        10980    10985       +5     
+ Misses       1428     1426       -2     

Flag	Coverage Δ
unittests-Project-Experimental	88.23% <100.00%> (-0.25%)	⬇️
unittests-Project-Stable	88.35% <100.00%> (+0.05%)	⬆️

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 3 files with indirect coverage changes

[Kielek]

Please check also https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/22649769411/job/65646261419?pr=6931

test/OpenTelemetry.Api.Tests/BaggageTests.cs [FAIL]
  Non-ASCII character found at Ln:34 b'            new("a \xe2\x9c\x85", "B% \xf0\x9f\x92\xbc"),\n'
  Non-ASCII character found at Ln:41 b'        baggage = baggage.SetBaggage("a \xe2\x9c\x85", "B% \xf0\x9f\x92\xbc");\n'
  Non-ASCII character found at Ln:55 b'        Assert.Equal("B% \xf0\x9f\x92\xbc", Baggage.Current.GetBaggage("a \xe2\x9c\x85"));\n'

[Kielek]

I checked also v1.54.0 version - only redactional changes related to the scope of PR. open-telemetry/opentelemetry-specification@v1.36.0...v1.54.0#diff-3d2a0e870c26fa9f2827cc33bd44f1b56939c65a35f55d5d988ebfd6a8d08c82
there is more stuff related to env. var. but it is out of the scope.

[github-actions]

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