fix(diff): compare MaxLength, not MinLength, in CheckStringTypeChanges (#253)

CheckStringTypeChanges passed type1.MinLength and type2.MinLength to
the MaxLength comparison. diff.Compare reported every minLength change
a second time as MaxLength, and never reported a maxLength change, so
narrowing maxLength went unflagged.

Add TestCheckStringTypeChanges, which fails on the old line.

Signed-off-by: Mike Minicki <martel@post.pl>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: martel

diff/checks.go

@@ -225,7 +225,7 @@ func CheckStringTypeChanges(diffs []TypeDiff, type1, type2 *spec.SchemaProps) []
 		type2.Type[0] == StringType {
 		minLengthDiffs := CompareIntValues("MinLength", type1.MinLength, type2.MinLength, NarrowedType, WidenedType)
 		diffs = append(diffs, minLengthDiffs...)
-		maxLengthDiffs := CompareIntValues("MaxLength", type1.MinLength, type2.MinLength, WidenedType, NarrowedType)
+		maxLengthDiffs := CompareIntValues("MaxLength", type1.MaxLength, type2.MaxLength, WidenedType, NarrowedType)
 		diffs = append(diffs, maxLengthDiffs...)
 		if type1.Pattern != type2.Pattern {
 			diffs = addTypeDiff(diffs, TypeDiff{Change: ChangedType, Description: fmt.Sprintf("Pattern Changed:%s->%s", type1.Pattern, type2.Pattern)})

diff/checks_test.go

@@ -231,6 +231,60 @@ func Test_checkNumericTypeChanges(t *testing.T) {
 	}
 }
 
+func TestCheckStringTypeChanges(t *testing.T) {
+	tests := []struct {
+		name  string
+		type1 *spec.SchemaProps
+		type2 *spec.SchemaProps
+		want  []TypeDiff
+	}{
+		{
+			name:  "MaxLength added",
+			type1: &spec.StringProperty().SchemaProps,
+			type2: &spec.StringProperty().WithMaxLength(255).SchemaProps,
+			want:  []TypeDiff{{Change: AddedConstraint, Description: "MaxLength(255)"}},
+		},
+		{
+			name:  "MinLength and MaxLength added",
+			type1: &spec.StringProperty().SchemaProps,
+			type2: &spec.StringProperty().WithMinLength(1).WithMaxLength(255).SchemaProps,
+			want: []TypeDiff{
+				{Change: AddedConstraint, Description: "MinLength(1)"},
+				{Change: AddedConstraint, Description: "MaxLength(255)"},
+			},
+		},
+		{
+			name:  "MaxLength narrowed",
+			type1: &spec.StringProperty().WithMaxLength(1000).SchemaProps,
+			type2: &spec.StringProperty().WithMaxLength(255).SchemaProps,
+			want:  []TypeDiff{{Change: NarrowedType, Description: "MaxLength 1000->255"}},
+		},
+		{
+			name:  "MaxLength widened",
+			type1: &spec.StringProperty().WithMaxLength(255).SchemaProps,
+			type2: &spec.StringProperty().WithMaxLength(1000).SchemaProps,
+			want:  []TypeDiff{{Change: WidenedType, Description: "MaxLength 255->1000"}},
+		},
+		{
+			name:  "MaxLength removed",
+			type1: &spec.StringProperty().WithMaxLength(255).SchemaProps,
+			type2: &spec.StringProperty().SchemaProps,
+			want:  []TypeDiff{{Change: DeletedConstraint, Description: "MaxLength(255)"}},
+		},
+		{
+			name:  "MinLength changed with MaxLength unchanged",
+			type1: &spec.StringProperty().WithMinLength(1).WithMaxLength(255).SchemaProps,
+			type2: &spec.StringProperty().WithMinLength(2).WithMaxLength(255).SchemaProps,
+			want:  []TypeDiff{{Change: NarrowedType, Description: "MinLength 1->2"}},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, CheckStringTypeChanges(nil, tt.type1, tt.type2))
+		})
+	}
+}
+
 type compareValueCase struct {
 	name       string
 	fieldName  string