agentHost: enable subagent integration test for Claude real SDK (#317353)

agentHost: enable subagent test for Claude real SDK

- Refactor IRealSdkProviderConfig.subagentToolName (string) to
  subagentToolNames (readonly string[]) so providers that expose multiple
  subagent-kind tools can list them all.
- Claude registers both 'Task' and 'Agent' as subagent tools; the model
  may pick either. Listing both exempts both from the 'parent must not
  contain inner tool calls' assertion.
- Flip supportsSubagents to true for Claude now that subagent routing
  works end-to-end.