internal/mcp: unify json tag parsing

Do a thorough job of parsing json struct tags.

Move the code to a place where both mcp and jsonschema can use it.

Change-Id: I6c86308834706b12a1e22cd96ae120590f49d3f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678757
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

jsonschema/validate_test.go

@@ -259,26 +259,6 @@ func TestStructInstance(t *testing.T) {
 	}
 }
 
-func TestJSONName(t *testing.T) {
-	type S struct {
-		A int
-		B int `json:","`
-		C int `json:"-"`
-		D int `json:"-,"`
-		E int `json:"echo"`
-		F int `json:"foxtrot,omitempty"`
-		g int `json:"golf"`
-	}
-	want := []string{"A", "B", "", "-", "echo", "foxtrot", ""}
-	tt := reflect.TypeFor[S]()
-	for i := range tt.NumField() {
-		got, _ := jsonName(tt.Field(i))
-		if got != want[i] {
-			t.Errorf("got %q, want %q", got, want[i])
-		}
-	}
-}
-
 func mustMarshal(x any) json.RawMessage {
 	data, err := json.Marshal(x)
 	if err != nil {