add node-embedded custom tools

[MuJiongan]

Summary

• add a NodeTool base class that automatically registers lowercase custom tool classes embedded in each node's Python source
• dispatch custom tools through the existing quoted-name interfaces: ctx.agent(tools=["lookup_order"]) and ctx.tools.lookup_order(...)
• keep registries node-local so concurrently executing nodes cannot collide
• restore embedded tools from frozen node source when continuing an agent conversation
• document the contract and add coverage for validation, isolation, subprocess execution, tracing, and continuations

Why

The orchestrator needs to define task-specific tools within a node without adding another node field or database schema. Embedding the declaration in the existing Python source keeps workflow snapshots and import/export behavior unchanged, while node-local registration avoids process-global tool collisions.

Impact

A node can now declare a lowercase NodeTool subclass alongside run(inputs, ctx). Its class name becomes the tool name automatically. Schemas, signatures, casing, and collisions with native or MCP tools are validated when the node source loads.

Validation

• python -m compileall -q app tests
• full backend test suite: 339 passed
• git diff --check