internal/mcp/jsonschema: validate defaults

Optionally validate a schema's defaults when resolving it.

We simply walk the tree of schemas, validating each locally. This can't
handle dynamic references, which require maintaing the stack from the
root. It seems unlikely that a default would require one, however.

Since there may be other options to Resolve, use an options struct
to future-proof it.

Also, fix a bug: you can't resolve a schema twice.

Change-Id: I374c582d9cc49d66d4ed56233d1070b8b3263bdf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676395
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/schema.go

@@ -182,7 +182,9 @@ type anchorInfo struct {
 // String returns a short description of the schema.
 func (s *Schema) String() string {
 	if s.uri != nil {
-		return s.uri.String()
+		if u := s.uri.String(); u != "" {
+			return u
+		}
 	}
 	if a := cmp.Or(s.Anchor, s.DynamicAnchor); a != "" {
 		return fmt.Sprintf("%q, anchor %s", s.base.uri.String(), a)