docs(TDD): Clarify FastAPI server execution and fix markdown for Phase 4

docs(TDD): Clarify FastAPI server execution and fix markdown for Phase 4

This commit enhances the Technical Design Document (Design Docs/TDD.md) by providing explicit instructions for running the FastAPI server, pertinent to the completion of Phase 4 ("Green Implementation for FastAPI"). It also includes a minor markdown syntax correction.

Key changes:
- Added a dedicated markdown block in Phase 4 with the precise command `uvicorn app_fastapi.app:app --host 127.0.0.1 --port 8000` and context for running the FastAPI application. This makes the instruction more prominent and user-friendly.
- Removed the previous, less visible comment `# Uvicorn will run this: uvicorn app_fastapi.app:app --port 8000` from within the FastAPI Python code example to avoid redundancy and improve clarity.
- Corrected markdown code block specifiers (from ``` to ````) around the FastAPI example and the new instruction block to ensure correct rendering of the document.

These changes make it easier for anyone following the TDD to manually start and test the FastAPI server as intended at the end of Phase 4, before proceeding to the benchmarking in Phase 5.