Improve connection reset handling during ServiceAccountToken rotation…

[ciarams87]

Proposed changes

Cherry-picked from #3905

Problem: During ServiceAccountToken rotation, nginx-gateway-fabric was sometimes experiencing deadlocks due to the Subscribe method incorrectly intercepting initial configuration operations after a ServiceAccountToken rotation and incorrect signaling broadcast completion

Solution: Implemented a pendingRequest tracking mechanism to distinguish between initial setup operations and broadcast operations.
Testing: Describe any testing that you did.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Fixed a bug where the subscribe method was incorrectly intercepting initial configuration operations after a ServiceAccountToken rotation and signaling broadcast completion

[codecov]

Codecov Report

❌ Patch coverage is 56.75676% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.88%. Comparing base (50bed80) to head (5a0b8e4).
⚠️ Report is 1 commits behind head on release-2.1.

Files with missing lines	Patch %	Lines
internal/controller/nginx/agent/file.go	25.00%	8 Missing and 1 partial ⚠️
internal/controller/nginx/agent/command.go	58.82%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-2.1    #3932      +/-   ##
===============================================
- Coverage        86.95%   86.88%   -0.07%     
===============================================
  Files              128      128              
  Lines            16001    16029      +28     
  Branches            62       62              
===============================================
+ Hits             13913    13927      +14     
- Misses            1921     1933      +12     
- Partials           167      169       +2     

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