internal/mcp/jsonschema: generalize error wrapping

Add a wrapf function to simplify defers that wrap errors.

Change-Id: If4bedec7d3f56f609d3ca4620aebe7637a59f6bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/util.go

@@ -282,3 +282,10 @@ func assert(cond bool, msg string) {
 		panic("assertion failed: " + msg)
 	}
 }
+
+// wrapf wraps *errp with the given formatted message if *errp is not nil.
+func wrapf(errp *error, format string, args ...any) {
+	if *errp != nil {
+		*errp = fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), *errp)
+	}
+}