chore: add changeset

Author: hassan123789

.changeset/oauth-resource-metadata-url.md

@@ -0,0 +1,11 @@
+---
+"@modelcontextprotocol/client": patch
+---
+
+Fix OAuth resource metadata URL persistence across browser redirects
+
+Added two optional methods to `OAuthClientProvider`:
+- `saveResourceMetadataUrl(url: URL)`: Saves the URL before redirect
+- `resourceMetadataUrl()`: Loads the saved URL after redirect
+
+This fixes token exchange failures in browser OAuth flows where the resource metadata URL discovered from the WWW-Authenticate header was lost during redirects.