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/infer.go

@@ -102,7 +102,7 @@ func typeSchema(t reflect.Type, seen map[reflect.Type]*Schema) (*Schema, error)
 	case reflect.Struct:
 		s.Type = "object"
 		// no additional properties are allowed
-		s.AdditionalProperties = &Schema{Not: &Schema{}}
+		s.AdditionalProperties = falseSchema()
 
 		for i := range t.NumField() {
 			field := t.Field(i)