[API] Normalize empty environment variable names

[martincostello]

open-telemetry/opentelemetry-specification#5163.

Changes

Normalize an empty value to _.

/cc @pellared

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 90.26%. Comparing base (0fd5949) to head (dc9906e).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7424      +/-   ##
==========================================
+ Coverage   89.84%   90.26%   +0.42%     
==========================================
  Files         276      279       +3     
  Lines       14611    15584     +973     
==========================================
+ Hits        13127    14067     +940     
- Misses       1484     1517      +33     

Flag	Coverage Δ
unittests-Project-Experimental	89.87% <100.00%> (+0.10%)	⬆️
unittests-Project-Stable	89.89% <100.00%> (+0.16%)	⬆️

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

Files with missing lines	Coverage Δ
.../Context/Propagation/EnvironmentVariableCarrier.cs	98.59% <100.00%> (+0.04%)	⬆️

... and 14 files with indirect coverage changes

[Copilot]

Pull request overview

Updates the experimental EnvironmentVariableCarrier key normalization logic in OpenTelemetry.Api to align with the updated environment variable carrier specification by ensuring empty propagation keys normalize to a valid environment variable name.

Changes:

• Normalize an empty key ("") to a single underscore ("_") in EnvironmentVariableCarrier.NormalizeKey.
• Update normalization documentation to explicitly describe the empty-key behavior.
• Extend unit tests to cover the empty-key normalization case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/OpenTelemetry.Api.Tests/Context/Propagation/EnvironmentVariableCarrierTests.cs	Adds coverage asserting NormalizeKey("") returns "_".
src/OpenTelemetry.Api/Context/Propagation/EnvironmentVariableCarrier.cs	Implements empty-key normalization and updates remarks; ensures empty keys are not treated as already-normalized.
src/OpenTelemetry.Api/CHANGELOG.md	Documents the experimental behavior change for empty key normalization.

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