[API] Handle empty tracestate value

[martincostello]

Found by Claude Code scan of the repository.

Changes

Avoid an internal IndexOutOfRangeException if a tracestate value was empty once trimmed.

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

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.86%. Comparing base (7dbd68d) to head (b33a0fa).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...lemetry.Api/Context/Propagation/TraceStateUtils.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7756      +/-   ##
==========================================
+ Coverage   91.85%   91.86%   +0.01%     
==========================================
  Files         337      337              
  Lines       18399    18399              
==========================================
+ Hits        16901    16903       +2     
+ Misses       1498     1496       -2     

Flag	Coverage Δ
unittests-Project-Experimental	91.98% <66.66%> (+<0.01%)	⬆️
unittests-Project-Stable	91.97% <66.66%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...lemetry.Api/Context/Propagation/TraceStateUtils.cs	91.57% <66.66%> (-1.06%)	⬇️

... and 2 files with indirect coverage changes

[Copilot]

🟢 Approval recommended

The fix is correct and adequately tested; the remaining feedback is cosmetic.

Pull request overview

Prevents empty trimmed tracestate values from causing internal exceptions.

Changes:

• Rejects empty values safely and clears partial results.
• Adds regression coverage for whitespace-only values.
• Documents the fix.

File summaries

File	Description
TraceStateUtils.cs	Safely validates empty values.
TracestateUtilsTests.cs	Adds regression cases.
CHANGELOG.md	Records the behavioral fix.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

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