Fix how status API URL is determined when both stub status and NGINX Plus APIs are configured

[dhurley]

Proposed changes

Fix how status API URL is determined when both stub status and NGINX Plus APIs are configured.

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

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[codecov-commenter]

Codecov Report

Patch coverage: 22.98% and project coverage change: -0.46% ⚠️

Comparison is base (7cc0918) 67.48% compared to head (4812798) 67.03%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #433      +/-   ##
==========================================
- Coverage   67.48%   67.03%   -0.46%     
==========================================
  Files         113      113              
  Lines       12777    12838      +61     
==========================================
- Hits         8623     8606      -17     
- Misses       3584     3662      +78     
  Partials      570      570              

Files Changed	Coverage Δ
src/core/nginx.go	45.86% <0.00%> (-0.54%)	⬇️
...or/github.com/nginx/agent/sdk/v2/config_helpers.go	63.49% <26.31%> (-5.41%)	⬇️

... and 3 files with indirect coverage changes

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