fix: handle IPv6 brackets and port in X-Forwarded-For parsing

[guoyangzhen]

Summary

Fixes #4572 — validateHeader fails to parse X-Forwarded-For values with IPv6 brackets or port numbers.

Problem

Some reverse proxies (IIS, cloud load balancers) send X-Forwarded-For with:

• IPv6 in brackets: [240e:318:2f4a:de56::240]
• Port numbers: 192.168.8.39:38792
• Both: [240e:318:2f4a:de56::240]:38792

net.ParseIP returns nil for all of these, causing ClientIP() to fall back to RemoteAddr.

Fix

Use net.SplitHostPort to properly strip brackets and port, with fallback to manual bracket removal for bare bracketed IPv6.

Test Cases

"192.168.8.39"                    → 192.168.8.39 ✅
"[::1]"                           → ::1 ✅
"192.168.8.39:38792"              → 192.168.8.39 ✅
"[::1]:8080"                      → ::1 ✅
"[240e:318:2f4a:de56::240]"       → 240e:318:2f4a:de56::240 ✅
"[240e:318:2f4a:de56::240]:38792" → 240e:318:2f4a:de56::240 ✅

Fixes #4572

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.34%. Comparing base (3dc1cd6) to head (ec78927).
⚠️ Report is 275 commits behind head on master.

Files with missing lines	Patch %	Lines
gin.go	40.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4584      +/-   ##
==========================================
- Coverage   99.21%   98.34%   -0.87%     
==========================================
  Files          42       48       +6     
  Lines        3182     3143      -39     
==========================================
- Hits         3157     3091      -66     
- Misses         17       41      +24     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.33% <40.00%> (?)
-tags go_json	98.20% <40.00%> (?)
-tags nomsgpack	98.25% <40.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.28% <40.00%> (?)
go-1.26	98.34% <40.00%> (?)
macos-latest	98.28% <40.00%> (-0.94%)	⬇️
ubuntu-latest	98.34% <40.00%> (-0.87%)	⬇️

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