fix(binding): handle non-JSON string values in form binding for custom struct types

[mango766]

Summary

Fixes #2510

When a struct field implements json.Unmarshaler and is used with form binding (e.g. application/x-www-form-urlencoded), the form value is passed directly to json.Unmarshal. Since form values are plain strings (like 2020/09/23 13:20:49), not valid JSON, the unmarshal fails with an error like invalid character '/' after top-level value.

This PR adds a fallback: if the initial json.Unmarshal fails, it retries with the value wrapped in double quotes, treating it as a JSON string. This lets custom types implementing json.Unmarshaler (e.g. custom datetime parsers) work correctly with form binding, while keeping existing JSON-based form values working as before.

Changes

• Extract unmarshalFieldAsJSON helper in binding/form_mapping.go that handles the quote-and-retry logic
• Apply it to both reflect.Struct and reflect.Map fallback paths in setWithProperType
• Add tests for custom json.Unmarshaler struct types with form binding

Pull Request Checklist

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md. (Not applicable — this is a bug fix)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (cb5463e).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4585      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3145      -37     
==========================================
- Hits         3157     3094      -63     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <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	98.37% <100.00%> (?)
go-1.26	98.37% <100.00%> (?)
macos-latest	98.37% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <100.00%> (-0.84%)	⬇️

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