feat(scene): route Scene MCP commands to PinK editor runtimes

[YuchenWell]

Problem

Scene MCP calls always used the cocos-cli-owned Scene Worker, even when invoked from PinK. This created a second Scene Runtime, so file changes could be saved without updating the scene tree or Inspector displayed in PinK.

Solution

• Introduce a pluggable SceneCommandBackend.
• Preserve WorkerSceneCommandBackend for standalone cocos-cli usage.
• Add a PinK backend that routes Scene RPC through host-provided callbacks.
• Scope route capabilities and operation IDs to each MCP tool call with AsyncLocalStorage.
• Complete each multi-RPC tool operation exactly once.

Compatibility and safety

• Standalone cocos-cli continues using the existing Scene Worker.
• PinK requests fail closed when the route context is missing, expired, or invalid.
• Failed PinK requests never fall back to the Scene Worker.
• Route capability tokens remain isolated between concurrent calls and are not logged.