chore: add changeset for google-duration format registration

Author: norberto

.changeset/register-google-duration-format.md

@@ -0,0 +1,9 @@
+---
+'@modelcontextprotocol/core-internal': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+---
+
+Register `google-duration` format on default AJV instances to silence "unknown format" warnings.
+
+Firebase MCP and other Google Cloud API MCP servers use the `google-duration` format in their JSON Schemas (values like `"3600s"`, `"1.5s"`). AJV logs `unknown format "google-duration" ignored` warnings for each schema that references it, creating noisy startup output. The format is registered with a regex matching Google's Duration proto format: a number followed by a time unit suffix (`s`, `ms`, `us`, `ns`, `m`, `h`, `d`).