Return extracted SpanContext when it is sampled but invalid

[thswlsqls]

Fixes #5339

Description

• The OpenTracing compatibility spec requires extract to return a SpanContext Shim if the context is valid, is sampled, or carries non-empty Baggage. Propagation.extractTextMap implemented only the isValid and Baggage terms, so an invalid but sampled context was dropped as null.
• Adds the missing isSampled term to match the spec code block. Invalid but sampled contexts support jaeger-debug-id headers, which force propagation of debug information.
• An invalid, unsampled context with empty Baggage still returns null.
• The spec condition was settled in OpenTracing Shim: Allow invalid but sampled SpanContext to be returned. opentelemetry-specification#3471; an earlier attempt ([OpenTracing Shim] Support TraceFlags-only propagation without parent span #5380) was closed as stale while that was pending.

Testing done

• Added TracerShimTest#extract_invalidButSampledSpanContext: a test propagator extracts an invalid but sampled SpanContext, and extract returns a non-null, sampled SpanContextShim. Fails without the Propagation change.
• ./gradlew :opentracing-shim:check — 84 tests passed.
• Added a ## Unreleased CHANGELOG entry. Propagation is package-private and no public signature changed, so no apidiff update was generated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (09d6c17) to head (8db6342).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8634   +/-   ##
=========================================
  Coverage     91.64%   91.64%           
- Complexity    10348    10349    +1     
=========================================
  Files          1013     1013           
  Lines         27380    27382    +2     
  Branches       3218     3220    +2     
=========================================
+ Hits          25092    25094    +2     
  Misses         1558     1558           
  Partials        730      730           

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