fix: Non-standard X-Forwarded-For header content is not supported

[dusk5213]

This PR fixes the issue reported in #4572.

Changes include:

1. Support IPv6 addresses with brackets

• Previously, c.GetClientIP() could not recognize addresses like [240e:318:2f4a:de56::240]. They are now parsed correctly.

2. Support IPv4/IPv6 addresses with port numbers

• Fixed the issue where X-Forwarded-For headers containing ports, common in IIS, ARR, or cloud load balancer scenarios, could not be recognized, e.g.:

127.0.0.1:38792
[240e:318:2f4a:de56::240]:38792

3. Preserve original behavior

• Plain IPv4 and IPv6 addresses are still recognized as before.

4. Add regression tests

• Covers the above problem cases as well as normal cases to ensure the fix works and does not break existing functionality.
  This change only affects the parsing logic in validateHeader and does not impact other features.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (3dc1cd6) to head (e4e3634).
⚠️ Report is 268 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4575      +/-   ##
==========================================
- Coverage   99.21%   98.32%   -0.90%     
==========================================
  Files          42       48       +6     
  Lines        3182     3155      -27     
==========================================
- Hits         3157     3102      -55     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.30% <100.00%> (?)
-tags go_json	98.17% <100.00%> (?)
-tags nomsgpack	98.23% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.25% <100.00%> (?)
go-1.26	98.32% <100.00%> (?)
macos-latest	98.25% <100.00%> (-0.96%)	⬇️
ubuntu-latest	98.32% <100.00%> (-0.90%)	⬇️

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