chore: add changeset for clientMetadataUrl fail-fast validation

Author: rechedev9

.changeset/fix-validate-client-metadata-url.md

@@ -0,0 +1,10 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Fix: validate `clientMetadataUrl` at construction time (fail-fast)
+
+`OAuthClientProvider` implementations that supply a `clientMetadataUrl` now have it
+validated immediately when the transport is constructed rather than at the first auth
+attempt. Invalid URLs throw synchronously, making misconfiguration easier to catch
+during development instead of surfacing as a runtime error mid-flow.