jsonschema: emit properties:{} again — value receiver + no omitempty

MarshalJSON had drifted to a pointer receiver while Properties gained
`,omitempty`. Together those defeated the nil->empty-map fill: encoding/json
never invokes a pointer-receiver MarshalJSON on non-addressable values (map
entries, a Definition passed by value), and omitempty drops the empty map even
when it does. Nested schemas therefore lost the "properties":{} that OpenAI
requires on object schemas — the exact bug #419/#434 fixed.

Value receiver restores the guarantee for map entries and by-value marshals;
dropping omitempty lets the filled empty map through. TestDefinition_MarshalJSON
(got and gotPtr) passes again.

Co-authored-by: Hanzo Dev <[email protected]>

jsonschema/json.go

@@ -35,7 +35,7 @@ type Definition struct {
 	// one element, where each element is unique. You will probably only use this with strings.
 	Enum []string `json:"enum,omitempty"`
 	// Properties describes the properties of an object, if the schema type is Object.
-	Properties map[string]Definition `json:"properties,omitempty"`
+	Properties map[string]Definition `json:"properties"`
 	// Required specifies which properties are required, if the schema type is Object.
 	Required []string `json:"required,omitempty"`
 	// Items specifies which data type an array contains, if the schema type is Array.
@@ -48,7 +48,7 @@ type Definition struct {
 	AdditionalProperties any `json:"additionalProperties,omitempty"`
 }
 
-func (d *Definition) MarshalJSON() ([]byte, error) {
+func (d Definition) MarshalJSON() ([]byte, error) {
 	if d.Properties == nil {
 		d.Properties = make(map[string]Definition)
 	}
@@ -56,7 +56,7 @@ func (d *Definition) MarshalJSON() ([]byte, error) {
 	return json.Marshal(struct {
 		Alias
 	}{
-		Alias: (Alias)(*d),
+		Alias: (Alias)(d),
 	})
 }