internal/mcp: meta and progress token

Meta is a struct that contains a map[string]any, and a ProgressToken
field.

Each Params and Result type has a Meta as a value field.

Meta marshals and unmarshals specially, so the ProgressToken field
becomes part of the map. The marshalling code is generalized in
anticipation of every Params and Result growing a map of additional
properties.

Change-Id: I6d2f34270d33501c96437f24f91ef3e51d20d955
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674956
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

jsonschema/validate_test.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -171,6 +172,26 @@ 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])
+		}
+	}
+}
+
 // loadRemote loads a remote reference used in the test suite.
 func loadRemote(uri *url.URL) (*Schema, error) {
 	// Anything with localhost:1234 refers to the remotes directory in the test suite repo.