Bound NGINX Plus API requests with a timeout

[CVanF5]

The NGINX Plus API HTTP client had no timeout, so a Plus API call that never responds (e.g. an unresponsive Plus API socket) blocks the agent's message bus indefinitely. The agent then never sends a DataPlaneResponse, leaving the management plane waiting on it.

• Set the HTTP client timeout from Client.HTTP.Timeout (falling back to a default when unset), following the existing datasource/config pattern.
• Timeout the unix socket dial with net.Dialer.Timeout and pass the per-request context through, following internal/grpc/proxy_dialer.go, so a stuck socket cannot block the dial.
• Return the original error from createPlusAPIError when it cannot be parsed into the structured Plus API format (e.g. a timeout). Previously it returned nil, so a failed apply was reported to the management plane as COMMAND_STATUS_OKinstead of a failure.

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
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.92%. Comparing base (006f54b) to head (53ffe37).

Files with missing lines	Patch %	Lines
internal/nginx/nginx_service.go	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1775      +/-   ##
==========================================
+ Coverage   84.89%   84.92%   +0.02%     
==========================================
  Files         105      105              
  Lines       13656    13663       +7     
==========================================
+ Hits        11593    11603      +10     
+ Misses       1540     1538       -2     
+ Partials      523      522       -1     

Files with missing lines	Coverage Δ
internal/nginx/nginx_service.go	61.41% <69.23%> (+2.43%)	⬆️

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