Add email validation for empty and whitespace-only values

[Copilot]

Addresses missing input validation on email parameters in the signup and unregister endpoints.

Changes

• Input validation: Added if not email.strip() checks to reject empty or whitespace-only emails in both /activities/{activity_name}/signup and /activities/{activity_name}/unregister endpoints
• Error handling: Returns 400 Bad Request with message "Email cannot be empty or whitespace-only"
• Test coverage: Added 4 test cases covering empty and whitespace-only email validation for both endpoints

@app.post("/activities/{activity_name}/signup")
def signup_for_activity(activity_name: str, email: str):
    """Sign up a student for an activity"""
    # Validate email is not empty or whitespace-only
    if not email.strip():
        raise HTTPException(status_code=400, detail="Email cannot be empty or whitespace-only")
    
    # ... rest of validation

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