test binding, add form_test.go for form bindings

[miladev95]

Add binding/form_test.go with unit tests for Form, FormPost, and FormMultipart.
Tests cover:
names, basic POST binding, multipart binding using existing helper, and ensuring Form ignores non-multipart ParseMultipartForm error.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (3dc1cd6) to head (d4e9693).
⚠️ Report is 229 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4487      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     2987     -195     
==========================================
- Hits         3157     2957     -200     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.98% <ø> (?)
-tags go_json	98.92% <ø> (?)
-tags nomsgpack	98.98% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <ø> (?)
go-1.25	98.99% <ø> (?)
macos-latest	98.99% <ø> (-0.22%)	⬇️
ubuntu-latest	98.99% <ø> (-0.22%)	⬇️

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[Nurysso]

The test looks good but Gin already has similar tests covering this functionality in binding/multipart_form_mapping_test.go

If you'd like to contribute, consider enhancing the existing test file instead of adding a new one.