Add toString to ComposableAnnotatingSampler

[thswlsqls]

Fixes #8644

Description

• ComposableAnnotatingSampler was the only one of the seven ComposableSampler implementations without a toString() override that returns getDescription().
• ComposableParentThresholdSampler builds its description by concatenating its root sampler, so composing it with an annotating sampler surfaced an Object hash (ComposableAnnotatingSampler@1a2b3c) in the observable getDescription() output.
• Added the 3-line override, matching the six sibling samplers.

Testing done

• Added ComposableAnnotatingSamplerTest#descriptionInComposedSampler asserting the composed parentThreshold(annotating(...)) description contains the real annotating description and no @ object hash — RED before the fix, GREEN after.
• ./gradlew :sdk-extensions:incubator:check — passed.
• Package-private class in an -alpha module; no docs/apidiffs/ change.

[opentelemetry-pr-dashboard]

Pull request dashboard status

Status last refreshed: 2026-07-20 21:23:42 UTC.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (09d6c17) to head (be9bfd3).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8645   +/-   ##
=========================================
  Coverage     91.64%   91.64%           
- Complexity    10348    10349    +1     
=========================================
  Files          1013     1013           
  Lines         27380    27381    +1     
  Branches       3218     3218           
=========================================
+ Hits          25092    25093    +1     
  Misses         1558     1558           
  Partials        730      730           

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