internal/mcp/jsonschema: support validating structs

The instance passed to Resolved.Validate can be a struct, or can
contain structs. A struct is treated like the JSON object that it
marshals to.

Also support pointers to any depth. That is, if an instance is a **int,
then Validate will treat it like an int (or nil). That won't happen if
the instance is unmarshaled into an `any`, but may if it's unmarshaled
into a struct.

Change-Id: I39f9af58028bd779887495754519615cd5dfb6c8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670395
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/schema.go

@@ -121,6 +121,11 @@ type Schema struct {
 	patternProperties map[*regexp.Regexp]*Schema
 }
 
+// falseSchema returns a new Schema tree that fails to validate any value.
+func falseSchema() *Schema {
+	return &Schema{Not: &Schema{}}
+}
+
 // String returns a short description of the schema.
 func (s *Schema) String() string {
 	if s.ID != "" {
@@ -182,7 +187,7 @@ func (s *Schema) UnmarshalJSON(data []byte) error {
 			*s = Schema{}
 		} else {
 			// false is the schema that validates nothing.
-			*s = Schema{Not: &Schema{}}
+			*s = *falseSchema()
 		}
 		return nil
 	}