Fix fieldName to strip JSON tag options, matching JSONSchema

[cinar]

Problem

checker.go's fieldName uses a field's json tag value verbatim for the CheckErrors map key:

if jsonTag, ok := field.Tag.Lookup("json"); ok {
	name = jsonTag
}

A field tagged json:"email,omitempty" produces the error key "email,omitempty" instead of "email". A field tagged json:"-" produces the literal key "-".

schema.go's jsonPropertyName already handles this correctly (strips ,omitempty-style options via strings.Cut, treats "-" as "no JSON name"), so CheckStruct's error keys and JSONSchema's property names disagreed for any field with tag options — verified this before writing the fix.

Fix

fieldName now calls the existing jsonPropertyName helper instead of duplicating (and getting wrong) its own parsing. When jsonPropertyName reports false (the json:"-" case), fieldName falls back to the Go field name — a CheckErrors key always needs some name, whereas a schema property can simply be omitted for such a field, so the two call sites intentionally handle that case differently.

Testing

• Added TestCheckStructCustomNameWithOptions (json:"name,omitempty" → key "name") and TestCheckStructJSONIgnoredFieldFallsBackToFieldName (json:"-" → key "Name").
• Full suite passes, 100% coverage maintained.
• go vet, gosec clean.

Fixes #194

🤖 Generated with Claude Code

https://claude.ai/code/session_01FLdVmP5daHiknrTW4Geh2i

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (05bb006) to head (4005bfa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #219   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           52        52           
  Lines          896       896           
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  Hits           896       896           

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