feat(scene): support host-provided Scene command routing

[YuchenWell]

Problem

Scene commands were always sent to the cocos-cli-owned Scene Worker. Embedded hosts could not route commands to their own Scene Runtime, potentially creating two independent runtime states.

Changes

• Add the pluggable ISceneCommandProvider.
• Preserve WorkerSceneCommandProvider for standalone usage.
• Expose provider registration through the Scene facade.
• Add isolated MCP tool-call contexts with operation IDs and keyed finalizers.
• Prevent stale Worker callbacks from clearing newer providers.
• Create scene assets through Asset Manager instead of Scene RPC.
• Export Scene and MCP type declarations.

Compatibility

• Standalone cocos-cli continues using the existing Scene Worker.
• Provider failures are propagated without fallback.
• Concurrent MCP calls keep their contexts isolated.

[bofeng-song]

Related issue: https://zentao.sud.center/index.php?m=bug&f=view&bugID=974