docs: add spec schema link to migration guide

Reference the MCP specification schema to prove that Resource URI
change from AnyUrl to str is spec-compliant.

Author: felixweinberger

docs/migration.md

@@ -54,7 +54,7 @@ The `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters have been re
 
 ### Resource URI type changed from `AnyUrl` to `str`
 
-The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the MCP specification which defines URIs as plain strings without validation. This change allows relative paths like `users/me` that were previously rejected.
+The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the [MCP specification schema](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/draft/schema.ts) which defines URIs as plain strings (`uri: string`) without strict URL validation. This change allows relative paths like `users/me` that were previously rejected.
 
 **Before (v1):**