Add Mermaid Support in Fenced Code Blocks

[Sam Clark (28pins)]

Summary

• Added mermaid support using beautiful-mermaid-swift
• For compatibility, rendering can be disabled using MermaidConfig.disabled
• MermaidConfig offers theme customization options (inherits directly from package, VS Code based)

Validation

Add tests, check demos, stress test kitchen sink demo

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

Pull request overview

Copilot reviewed 15 out of 16 changed files in this pull request and generated 3 comments.

[Sam Clark (28pins)]

Copilot Fix the code for all comments in this review thread.

When a review comment includes a suggested change, apply the suggestion exactly.

Do not make changes beyond what is described in the linked review thread.