feat(context): unwrap joined errors in Context.Error()

[Vierblatt]

Summary

• Context.Error() now detects errors implementing Unwrap() []error (like those from errors.Join) and stores each unwrapped error as an individual entry in c.Errors
• Preserves *gin.Error type metadata when a joined error contains one
• Falls through to the normal single-error path when Unwrap() returns an empty slice

Motivation

Fixes #4237. When errors.Join(e1, e2) is passed to c.Error(), the joined error was stored as a single entry. This produced confusing output in errorMsgs.String() where the joined error's own \n-separated format mixed with Gin's numbered format:

Error #01 gin error
Error #02 service error
store error          <--- part of a joined error, not numbered
Error #03 other error

Now each joined error becomes its own numbered entry.

Test plan

• Basic errors.Join(err1, err2) unwraps into two entries
• Nested errors.Join recursively unwraps
• *gin.Error type preserved inside a join
• Empty Unwrap() []error degrades to single wrapped entry
• All existing tests pass
• Race detector clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3dc1cd6) to head (5228977).
⚠️ Report is 292 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4729      +/-   ##
==========================================
- Coverage   99.21%   98.33%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3178       -4     
==========================================
- Hits         3157     3125      -32     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.33% <100.00%> (?)
go-1.26	98.33% <100.00%> (?)
macos-latest	98.33% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.33% <100.00%> (-0.89%)	⬇️

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