Training agent: capability_discovery scenario fails creative capabilities validation

[bokelley]

Bug

Running the capability_discovery storyboard against the public test agent fails:

capability_discovery → core track → FAIL
  ✗ capability_discovery — Validate creative capabilities: failed

Agent: https://test-agent.adcontextprotocol.org/mcp

The get_adcp_capabilities response from the training agent doesn't pass the capability_discovery scenario's creative capabilities validation check in @adcp/client.

Expected

The training agent's capabilities response should pass all validators in the capability_discovery scenario.

Context

Discovered during storyboard validation (#1985). The capability_discovery storyboard was added to cover the only task without storyboard coverage.