[SwastidebDas]

Summary

Fixes #1373.

AI-generated Mermaid diagrams can fail to render when the language model wraps its response in Markdown code fences, even though the generation task completes successfully.

Changes

• Add a utility to remove mermaid Markdown code fences before passing the response to parseMermaidToExcalidraw.
• Preserve valid Mermaid responses without code fences.
• Display error messages in the AI diagram dialog instead of always showing a generic error.
• Add regression tests for fenced responses, unfenced responses, Windows-style line endings, and incomplete code fences.

Testing

• npx vitest run tests/integration/mermaidCodeFence.spec.mjs — 4 tests passed.
• ESLint checks passed for all three modified/added files.
• git diff --check passed.

Note: The full test suite was not verified successfully because the unrelated viewer-bundle.spec.mjs test timed out during its build step.