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

Including embedded struct inforamtion in error message.

Fixes #68941

Change-Id: I6a6f7d506104839a9a7cf1a2c3003272f5534a79
GitHub-Last-Rev: 717f680aca
GitHub-Pull-Request: golang/go#68966
Reviewed-on: https://go-review.googlesource.com/c/go/+/606956
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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},