Follow-up improvements for sandboxing mermaid diagrams

Follow-up improvements from !74414 (merged)

What does this MR do and why?

This implements a few suggestions from !74414 (merged) addressing

1. Suggestion: Read the path for iframe src from the backend, instead of hard coding it
2. Suggestion: Refactor warning text message to prevent injection attacks

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Same as !74414 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.