Implement Get Todo endpoint (GET /todos/:id)

[landoyjx]

Implement GET /todos/:id to retrieve a single Todo by its identifier. Response should be 200 OK with the Todo object, or 404 Not Found if the id does not exist.

Acceptance Criteria:

• Returns 200 and the Todo object for a valid existing id
• Returns 404 with JSON error for a non-existent id

Test Cases:

• Integration: GET /todos/:id with a valid id returns the correct Todo
• Integration: GET /todos/:id with a non-existent id returns 404
• Edge: GET /todos/:id with a malformed id format returns 404 (or 400 if framework-specific)