fix: route /victoriametrics writes through vmproxy (PMM-15030)

[theTibi]

Summary

• In external VictoriaMetrics mode (PMM_VM_URL set), nginx still proxied /victoriametrics/ to local 127.0.0.1:9090/prometheus/, causing write-path failures.
• Route /victoriametrics/ through vmproxy (http://vmproxy/prometheus/) to align with /prometheus/api/v1 behavior.

Ticket

• PMM-15030

Test plan

• Brought up PMM test stack with merged compose files (docker-compose.yml + docker-compose.dev.yml) and external VM mode configured.
• Verified active config inside pmm-server:
  • location /victoriametrics/ { proxy_pass http://vmproxy/prometheus/; }
• Ran nginx config validation in container:
  • docker exec pmm-server nginx -t -> syntax is ok and test is successful
• Exercised write endpoint through PMM:
  • curl -k -u admin:admin -X POST https://127.0.0.1/victoriametrics/api/v1/write --data-binary testdata -> HTTP 204
• pmm-submodules (FB) PR: fix-pmm-15030 (FB) Percona-Lab/pmm-submodules#4340

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.21%. Comparing base (64d3111) to head (624d382).
⚠️ Report is 38 commits behind head on v3.

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #5323      +/-   ##
==========================================
+ Coverage   42.19%   42.21%   +0.02%     
==========================================
  Files         410      410              
  Lines       41914    41914              
==========================================
+ Hits        17686    17696      +10     
+ Misses      22441    22434       -7     
+ Partials     1787     1784       -3     

Flag	Coverage Δ
admin	34.89% <ø> (ø)
agent	49.24% <ø> (+0.07%)	⬆️
managed	40.74% <ø> (+<0.01%)	⬆️
vmproxy	72.09% <ø> (ø)

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