docs(sdk): document server version requirement and deployment-path gap for the card filter

Adds the minimum-server note to metadata_filters (older servers ignore the
unknown query parameter and return the full agent list) and syncs the
generated agent_card_metadata docstrings with the updated OpenAPI
description from scaleapi/scale-agentex#411.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: declan-scale

src/agentex/lib/utils/metadata_filters.py

@@ -13,6 +13,11 @@
     client.agents.list(
         agent_card_metadata=encode_metadata_filter({"permits_capable": True}),
     )
+
+The ``agent_card_metadata`` filter requires an Agentex server that includes
+scaleapi/scale-agentex#411. Older servers ignore the unknown query parameter
+and return the full unfiltered agent list rather than erroring, and the SDK's
+startup backend-contract check does not guard against this.
 """
 
 from __future__ import annotations