Storyboard coverage: get_adcp_capabilities

[bokelley]

Add a capability_discovery.yaml storyboard that calls get_adcp_capabilities and validates the response schema. Currently the only task with no storyboard coverage.

Discovered in #424.

[bokelley]

Done in adcontextprotocol/adcp#1985. Added capability_discovery.yaml storyboard covering get_adcp_capabilities with protocol schema validation. Training agent validation passes after adcontextprotocol/adcp#1995.