fix(baggage): do not percent-encode W3C baggage metadata

[arimu1]

Description

Fixes #6771.

The W3C Baggage propagator was percent-encoding and percent-decoding metadata (W3C properties) on inject/extract. That is not required by either the W3C Baggage spec or the OTel baggage API (metadata is an opaque string). Encoding breaks property key-value shape for other implementations after an extract→inject round-trip, e.g.:

SomeKey=SomeValue;ValueProp \t = \t PropVal
→ SomeKey=SomeValue;ValueProp%20%09%20%3D%20%09%20PropVal

Change: leave metadata untouched on inject and extract. Baggage entry values continue to be percent-encoded/decoded as before (aligned with the W3C requirement for list-member values).

This matches the approach used by opentelemetry-js (opaque pass-through).

Testing

• Updated W3CBaggagePropagatorTest.inject expectation (metadata not encoded)
• Added regression tests for W3C Baggage Propagator should not percent-encode metadata #6771:
  • inject_doesNotPercentEncodeMetadata
  • extract_metadataNotPercentDecoded
  • roundTrip_metadataPreservedOpaque
• Invalid percent sequences in metadata no longer drop the member (only invalid value encoding does)
• Fuzz tests: metadata generator excludes list separator , (safe without encoding)

Local:

./gradlew :api:all:test \
  --tests 'io.opentelemetry.api.baggage.propagation.W3CBaggagePropagatorTest' \
  --tests 'io.opentelemetry.api.baggage.propagation.W3CBaggagePropagatorFuzzTest' \
  --tests 'io.opentelemetry.api.baggage.propagation.BaggageCodecTest'

All green on Temurin 21.0.12 (macOS aarch64).

Compatibility note

This is a deliberate behavior change for interop with W3C-compliant / JS implementations. Callers that relied on Java-side percent-encoding of metadata will see raw metadata strings on the wire and on extract. Values are unchanged.

CNCF CLA: may need human sign-off if the CLA bot requests it.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: arimu1 / name: arimu1 (fb8a048)

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.48%. Comparing base (f8bd413) to head (fb8a048).

Files with missing lines	Patch %	Lines
.../api/baggage/propagation/W3CBaggagePropagator.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8682      +/-   ##
============================================
- Coverage     91.48%   91.48%   -0.01%     
  Complexity    10467    10467              
============================================
  Files          1021     1021              
  Lines         27694    27691       -3     
  Branches       3247     3247              
============================================
- Hits          25337    25333       -4     
- Misses         1615     1616       +1     
  Partials        742      742              

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