encoding/json: add embedded structs to the UnmarshalTypeError's Field

Including embedded struct inforamtion in error message.

Fixes #68941

doc/next/6-stdlib/99-minor/encoding/json/68941.md

@@ -0,0 +1 @@
+[UnmarshalTypeError.Field] now includes embedded structs to provide more detailed error messages.

src/encoding/json/decode.go

@@ -127,7 +127,7 @@ type UnmarshalTypeError struct {
 	Type   reflect.Type // type of Go value it could not be assigned to
 	Offset int64        // error occurred after reading Offset bytes
 	Struct string       // name of the struct type containing the field
-	Field  string       // the full path from root node to the field
+	Field  string       // the full path from root node to the field, include embedded struct
 }
 
 func (e *UnmarshalTypeError) Error() string {
@@ -701,7 +701,10 @@ func (d *decodeState) object(v reflect.Value) error {
 			if f != nil {
 				subv = v
 				destring = f.quoted
-				for _, i := range f.index {
+				if d.errorContext == nil {
+					d.errorContext = new(errorContext)
+				}
+				for i, ind := range f.index {
 					if subv.Kind() == reflect.Pointer {
 						if subv.IsNil() {
 							// If a struct embeds a pointer to an unexported type,
@@ -721,13 +724,16 @@ func (d *decodeState) object(v reflect.Value) error {
 						}
 						subv = subv.Elem()
 					}
-					subv = subv.Field(i)
+					if i < len(f.index)-1 {
+						d.errorContext.FieldStack = append(
+							d.errorContext.FieldStack,
+							subv.Type().Field(ind).Name,
+						)
 					}
-				if d.errorContext == nil {
+					subv = subv.Field(ind)
-					d.errorContext = new(errorContext)
 				}
-				d.errorContext.FieldStack = append(d.errorContext.FieldStack, f.name)
 				d.errorContext.Struct = t
+				d.errorContext.FieldStack = append(d.errorContext.FieldStack, f.name)
 			} else if d.disallowUnknownFields {
 				d.saveError(fmt.Errorf("json: unknown field %q", key))
 			}

src/encoding/json/decode_test.go

@@ -898,6 +898,19 @@ var unmarshalTests = []struct {
 		},
 	},
 
+	{
+		CaseName: Name(""),
+		in:       `{"Level1a": "hello"}`,
+		ptr:      new(Top),
+		err: &UnmarshalTypeError{
+			Value:  "string",
+			Struct: "Top",
+			Field:  "Embed0a.Level1a",
+			Type:   reflect.TypeFor[int](),
+			Offset: 10,
+		},
+	},
+
 	// issue 15146.
 	// invalid inputs in wrongStringTests below.
 	{CaseName: Name(""), in: `{"B":"true"}`, ptr: new(B), out: B{true}, golden: true},