Add Mermaid Diagram Rendering to Agent Pane

[sebas-developer]

Right now, Mermaid diagrams just show up as plain code blocks in the agent pane, which isn't very useful. Users can't actually see the diagrams they're trying to share or view.

The solution: rendering Mermaid diagrams directly in the chat interface instead of just showing code blocks:

• Detect ```mermaid code blocks
• Parse and render them as actual diagrams
• Fall back to code blocks if rendering fails
• Keep it secure (use strict mode)

Benefits

• Users can see diagrams immediately without copying to external tools
• Better communication for flows, processes, and technical concepts
• More visual and engaging chat experience

Example

graph TD
    A[Current] --> B[Shows code block]
    C[Proposed] --> D[Shows actual diagram]

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This should show as a real diagram, not text.

[SomeoneToIgnore]

#10696