feat(tools): add dynamic agent creation toolset

[evanlowe]

Summary

• add one built-in CreateAgentToolset exposing collect_resources and create_agents
• collect Skill Hub, AgentKit Skill Center, AgentKit knowledge bases, and VeADK built-in tools concurrently
• create multiple agents concurrently with LLM, sequential, parallel, loop, and capability-gated Workflow nodes
• support two orchestration layers, dynamic Skill materialization, read-only knowledge tools, VeADK built-in tools, and main-agent-authored Python tools
• keep built-in tool references separate from complete Python source authored by the main Agent
• isolate resource snapshots by invocation and release them after use
• render dedicated Studio conversation cards with resource accordions and fixed-height Agent result cards
• summarize Skill, knowledge, tool, and Sub Agent counts with Apps SDK UI hover popovers; preserve full custom Python source in tool details
• preserve loading, empty, source-error, partial-failure, and failed states without exposing implementation hints

Compatibility

• detect Workflow support from both the installed Google ADK version and runtime imports
• dynamically omit Workflow from the tool schema on Google ADK 1.x
• support both Volcengine and BytePlus credentials for Skill resources
• keep built-in tool discovery provider-independent

Validation

• pre-commit: passed
• frontend tests: 868 passed
• production frontend build: passed
• create-agent tests: 24 passed
• Impeccable detector: no findings
• local end-to-end example: built-in web_search, authored Python tool, and nested sequential Agent all created successfully

Current boundary

Python tool source is intentionally executed in the host process without sandboxing. Declared dependencies are validated against the current interpreter and are never installed automatically.