[API] Refactor slicing code

[martincostello]

Noticed while working on #7433.

Changes

Use IsEmpty and consume the slice rather than tracking an index.

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)

[Copilot]

Pull request overview

Refactors the tracestate parsing loop in TraceContextPropagator to consume the ReadOnlySpan<char> as it is parsed (using IsEmpty and slicing), instead of tracking a separate index. This is part of the API-layer W3C Trace Context propagation logic.

Changes:

• Updated the TryExtractMultipleTracestate loop to parse comma-separated members by advancing the span (tracestate = tracestate.Slice(...)) until empty.
• Simplified slice/index usage by calling IndexOf(',') on the remaining span rather than slicing from an external begin offset.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.98%. Comparing base (88fd1f0) to head (780e201).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7446   +/-   ##
=======================================
  Coverage   89.98%   89.98%           
=======================================
  Files         277      277           
  Lines       14624    14623    -1     
=======================================
  Hits        13159    13159           
+ Misses       1465     1464    -1     

Flag	Coverage Δ
unittests-Project-Experimental	89.92% <100.00%> (+0.01%)	⬆️
unittests-Project-Stable	89.92% <100.00%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
....Api/Context/Propagation/TraceContextPropagator.cs	91.25% <100.00%> (+0.26%)	⬆️

... and 2 files with indirect coverage changes