Improve connection reset handling during ServiceAccountToken rotation

[ciarams87]

Proposed changes

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:

• Initial config operations: Do not signal ResponseCh (prevents spurious broadcast signals)
• Broadcast operations: Only signal ResponseCh when there's a pending broadcast request

Also added additional logging to improve debugging

Testing: Updated unit tests, existing functional and conformance tests pass, and reran longevity (with the corresponding Agent fixes) which now works

Partially closes #3626

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 86.78%. Comparing base (5c3fc1b) to head (7fabec5).
⚠️ Report is 1 commits behind head on main.

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             @@
##             main    #3905      +/-   ##
==========================================
- Coverage   86.86%   86.78%   -0.09%     
==========================================
  Files         128      128              
  Lines       16519    16547      +28     
  Branches       62       62              
==========================================
+ Hits        14350    14361      +11     
- Misses       1990     2005      +15     
- Partials      179      181       +2     

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[bjee19]

Changes help a bunch in clarity, thanks!