Fix broken Mermaid diagram rendering in architecture documentation

[stevenkiwanukakafeero]

One of the Mermaid diagrams in the architecture documentation fails to render on GitHub.

Problem:
GitHub shows a Mermaid parse error near the class definition section.

Suggested solution:
Review the Mermaid syntax and correct the invalid class name or diagram formatting.

Importance:
Fixing the diagram improves readability of the architecture documentation and helps contributors understand the request processing flow.