Creates diagram elements from a Mermaid definition. Creates a new canvas if room_id is omitted, or adds to the existing canvas if room_id is provided. Preferred over add_elements for new diagrams and large changes to existing diagrams whenever the content can be represented in Mermaid. Supports: flowchart, graph, flowchart-elk, sequenceDiagram, classDiagram, classDiagram-v2, stateDiagram, stateDiagram-v2, erDiagram, journey, gantt, pie, gitGraph, mindmap, timeline, C4Context, C4Container, C4Component, C4Dynamic, C4Deployment, sankey, sankey-beta, quadrantChart, xychart, xychart-beta, requirement, requirementDiagram, kanban, architecture, block, block-beta, packet, packet-beta, radar-beta, treemap, info. Example: "flowchart TD\n A[Start] --> B{Decision}\n B -->|Yes| C[OK]\n B -->|No| D[Cancel]"