fix(harness): enable session capability overlays

[evanlowe]

Summary

• mount the existing AgentKit session capability routes in HarnessApp
• register the ADK runtime services required by capability overlays and /harness/run_sse
• expose agent introspection using the Harness app name
• add HTTP regression coverage for agent info and tool capability add/remove

Root cause

#742 configured session capability overlays only in create_agentkit_app(). Harness runtimes use the separate HarnessApp entry point, so those routes were never mounted there.

Testing

• uv run pytest -q tests/cloud/test_harness_app_http.py tests/integrations/agentkit/test_session_capabilities.py
• Ruff checks and formatting hooks
• hardcoded-secret pre-commit check
• local Studio verification of tool and skill overlays through /harness/run_sse