Fix TraceStateBuilder.remove corrupting the builder when the same key is removed twice

[thswlsqls]

Fixes #8612

Description

• ArrayBasedTraceStateBuilder.remove() decremented numEntries without checking whether the value was already null, so removing a key twice drove the counter below the real count.
• build() trusts numEntries in three places, so the drift leaks a {a=null} entry, silently drops live entries, or throws ArrayIndexOutOfBoundsException.
• Mirrors the sibling put() (same class, line 74-78), which already does this check; first removal is unchanged.
• No in-repo caller, but TraceStateBuilder is stable public API for external instrumentation and vendor propagators. The silent data loss contradicts the no-op contract of the remove(String) Javadoc ("if it is present") and of removeNotPresent(), and is reachable via reuseBuilder().
• Same defect shape as Fix PooledHashMap dropping live entries when an entry is removed during forEach #8499 (PooledHashMap dropping live entries).

Testing done

• Added TraceStateTest#removeTwice, #removeTwice_KeepsRemainingEntry, #removeTwice_KeepsRemainingEntries for one, two, and three entries. All three fail without the fix; 41 pass with it.
• ./gradlew :api:all:check — passed, 504 tests.
• No signature change (package-private class): apidiff unchanged.
• CHANGELOG.md ## Unreleased → ### API entry added.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.63%. Comparing base (63ebd94) to head (0b8dc2f).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8613   +/-   ##
=========================================
  Coverage     91.63%   91.63%           
- Complexity    10326    10327    +1     
=========================================
  Files          1013     1013           
  Lines         27295    27296    +1     
  Branches       3205     3206    +1     
=========================================
+ Hits          25011    25013    +2     
  Misses         1558     1558           
+ Partials        726      725    -1     

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