fix: preserve overview updates during concurrent config applies

[pooyanazad]

Config context updates were dropped whenever another apply was in progress. Keep the latest context per instance and publish it when watchers are re-enabled, as long as it still matches the completed apply.

Add coverage for matching and stale buffered contexts.

Fixes #1927

Proposed changes

When two identical config applies overlap, the watcher can discard the
configuration context from the first apply. The second apply then sees the
same cached context, so neither apply publishes NginxConfigUpdateTopic and
UpdateOverview is never sent.

This change buffers the latest configuration context for each instance while
a config apply is in progress. When watchers are re-enabled, the buffered
context is published only if it matches the completed apply context. Stale
contexts are discarded.

Testing

• go test -count=1 ./internal/watcher/...
• go test -race -count=1 ./internal/watcher

Checklist

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

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• Not applicable, no documentation changes are required
• Not applicable, this change is platform-independent

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[pooyanazad]

I have hereby read the F5 CLA and agree to its terms

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.97%. Comparing base (ac2827d) to head (2984cee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1938      +/-   ##
==========================================
+ Coverage   88.94%   88.97%   +0.02%     
==========================================
  Files         111      111              
  Lines       12413    12422       +9     
==========================================
+ Hits        11041    11052      +11     
+ Misses       1370     1368       -2     
  Partials        2        2              

Files with missing lines	Coverage Δ
internal/watcher/watcher_plugin.go	76.70% <100.00%> (+4.24%)	⬆️

... and 2 files with indirect coverage changes

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[pooyanazad]

Fixed the lint issues from the previous CI run and squashed the fixes into the original commit. Ready for another CI run and review.