fix: remove false-positive HTTPS_PROXY warning when HTTPS_PROXY is already set

[pranav-new-relic]

Summary

• The guided install was printing a warning "Please ensure the HTTPS_PROXY environment variable is set" even when HTTPS_PROXY was already set with an http:// URL (e.g. HTTPS_PROXY=http://localhost:8080)
• Root cause: the condition used a broken string-matching heuristic — it checked if HTTPS_PROXY contained "http" but not "https", which matches any http:// URL
• Fix: replaced the heuristic with a direct env var check — the warning now only fires when HTTP_PROXY is set but HTTPS_PROXY is absent (the actual misconfiguration the message was meant to flag)
• Replaced the non-asserting TestProxyNetwork (which only called t.Log) with TestProxyWarningCondition, which covers all four cases with real assertions

Test plan

• go test -tags unit ./internal/install/ -run TestProxyWarningCondition -v — all 4 subtests pass
• Manual: set HTTPS_PROXY=http://localhost:8080 and run guided install — warning should no longer appear
• Manual: set HTTP_PROXY=http://localhost:8080 (no HTTPS_PROXY) and run guided install — warning should still appear

Fixes NR-560830 — resolved via AI (Claude Code / Claude Sonnet 4.6)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 31.32%. Comparing base (4ed200d) to head (48dfa5f).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/install/command.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1839      +/-   ##
==========================================
+ Coverage   31.30%   31.32%   +0.01%     
==========================================
  Files         181      181              
  Lines       12551    12588      +37     
==========================================
+ Hits         3929     3943      +14     
- Misses       8478     8504      +26     
+ Partials      144      141       -3     

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