fix: support non-standard X-Forwarded-For header formats (#4572)

[larkwins]

Handle non-standard IP formats in X-Forwarded-For header that are commonly set by reverse proxies like IIS and cloud load balancers:

• IPv4 with port: '192.168.8.39:38792'
• IPv6 with brackets: '[240e:318:2f4a:de56::240]'
• IPv6 with brackets and port: '[240e:318:2f4a:de56::240]:38792'

Added parseNonstandardIP() helper function that uses net.SplitHostPort() and bracket stripping to extract the clean IP address before validation.

Closes #4572

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.44%. Comparing base (3dc1cd6) to head (7b53fa1).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4591      +/-   ##
==========================================
- Coverage   99.21%   98.44%   -0.77%     
==========================================
  Files          42       48       +6     
  Lines        3182     3152      -30     
==========================================
- Hits         3157     3103      -54     
- Misses         17       40      +23     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.43% <100.00%> (?)
-tags go_json	98.30% <100.00%> (?)
-tags nomsgpack	98.35% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.38% <100.00%> (?)
go-1.26	98.44% <100.00%> (?)
macos-latest	98.38% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.44% <100.00%> (-0.77%)	⬇️

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