fix(binding): support custom unmarshaler for slices & arrays

[crunchydosa123]

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• Open your pull request against the master branch.
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[crunchydosa123]

This PR aims to solve #4296 .

Root Cause

The trySetCustom function in binding/binding.go invokes:

v.UnmarshalParam(val)

However, since the method signature of UnmarshalParam does not include the field name, custom unmarshallers cannot attach field context to their errors.

Solution

This PR wraps the error returned by UnmarshalParam with a new structured error type:

type FieldError struct {
Field string
Err error
}

trySetCustom() is updated to append the field name when returning errors from BindUnmarshaler.

This ensures that the standard error propagation chain includes the field name, resulting in user-friendly and debuggable error messages.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.55%. Comparing base (3dc1cd6) to head (527f128).
⚠️ Report is 220 commits behind head on master.

Files with missing lines	Patch %	Lines
binding/form_mapping.go	75.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4435      +/-   ##
==========================================
- Coverage   99.21%   98.55%   -0.66%     
==========================================
  Files          42       44       +2     
  Lines        3182     2984     -198     
==========================================
- Hits         3157     2941     -216     
- Misses         17       28      +11     
- Partials        8       15       +7     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.54% <75.00%> (?)
-tags go_json	98.48% <75.00%> (?)
-tags nomsgpack	98.53% <75.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.55% <75.00%> (?)
go-1.25	98.55% <75.00%> (?)
macos-latest	98.55% <75.00%> (-0.66%)	⬇️
ubuntu-latest	98.55% <75.00%> (-0.66%)	⬇️

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