internal/mcp: validate tool input schemas

A tool now validates its input with its InputSchema.

Change schema inference to allow explicit nulls for fields of
pointer type.

We assume the schema has no external references. For example, the
following schema cannot be handled:

  {
    "$ref": "https://example.com/other.json"
  }

Schemas with internal references, like to a "$defs", are fine.

Change-Id: I6ee7c18c2c5cb609df0b22a66da986f7ea64bbe4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

jsonschema/util.go

@@ -270,7 +270,7 @@ func jsonType(v reflect.Value) (string, bool) {
 		return "string", true
 	case reflect.Slice, reflect.Array:
 		return "array", true
-	case reflect.Map:
+	case reflect.Map, reflect.Struct:
 		return "object", true
 	default:
 		return "", false