Make ThreadContext.stashAndMergeHeaders package-private

[cwperks]

Description

The ThreadContext class overly exposes many methods as public. Currently, stashAndMergeHeaders is publicly exposed, but its only usage is in testing. This PR changes the access modifier on this method to package-private to prevent this internally used method from being exposed publicly.

Related Issues

Related to #14931

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

❌ Gradle check result for 56bebf6: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for fadc5fe: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for fadc5fe: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for fadc5fe: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 2aba768: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 71.81%. Comparing base (1fe58b5) to head (2aba768).
Report is 6 commits behind head on main.

Files	Patch %	Lines
...l/concurrent/StashAndMergeHeadersFilterClient.java	0.00%	7 Missing ⚠️
.../org/opensearch/client/support/AbstractClient.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14983      +/-   ##
============================================
+ Coverage     71.78%   71.81%   +0.03%     
- Complexity    62694    62728      +34     
============================================
  Files          5160     5162       +2     
  Lines        294211   294364     +153     
  Branches      42553    42577      +24     
============================================
+ Hits         211212   211411     +199     
+ Misses        65599    65568      -31     
+ Partials      17400    17385      -15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cwperks]

Closing in favor of #15039