fix: make mcp filter type explicit in mcp_filter demo (fixes #644)

Update the mcp_filter demo to explicitly declare `type: mcp` on each
filter definition, matching the style of the http_filter demo which
uses explicit `type: http`. This makes the configuration more readable
and self-documenting instead of relying on the implicit default.

demos/filter_chains/mcp_filter/config.yaml

@@ -7,16 +7,13 @@ agents:
 filters:
   - id: input_guards
     url: http://localhost:10500
-    # type: mcp (default)
-    # transport: streamable-http (default)
-    # tool: input_guards (default - same as filter id)
+    type: mcp
   - id: query_rewriter
     url: http://localhost:10501
-    # type: mcp (default)
-    # transport: streamable-http (default)
-    # tool: query_rewriter (default - same as filter id)
+    type: mcp
   - id: context_builder
     url: http://localhost:10502
+    type: mcp
 
 model_providers:
   - model: openai/gpt-4o-mini