feat(studio): add dynamic Studio BFF tools

[zakahan]

Summary

• add a per-run Studio BFF tool channel between Studio and Runtime, with capability discovery, WebSocket transport, and HTTP/SSE fallback
• publish selected tool catalogs to Runtime before model inference and execute tool calls locally in Studio BFF
• replace Session Capability persistence with runtime opt-in StudioExternalToolset integration
• expose VeADK built-ins plus auto-discovered first-party Studio extensions, including current_time as a minimal template
• label selected custom tools as Studio Tool and rebuild the packaged WebUI assets

Validation

• uv run pre-commit run --all-files
• uv run pytest tests/frontend/server/studio_tools tests/cli/test_frontend_runtime_proxy.py tests/integrations/agentkit/test_studio_channel.py -q (72 passed)
• node --test --test-reporter=dot tests/*.test.mjs
• npm run build
• npm run test:webui-assets (83 files, 226 references)

Known risk

If a Session used a Studio Tool previously and the tool is deselected later, the current run correctly omits its schema, but historical functionCall/functionResponse events remain in ADK Session history. The model may infer the old tool name and emit another call, which Runtime rejects with Tool not found. The deselected tool is not executed and its schema is not persisted; filtering unavailable historical tool events or adding graceful recovery is left for follow-up.