input: Write a function to implement a simple chatbot using Python and Dialogflow that can answer questions about the current days of the week. The function should handle the detection of user intent and return the appropriate response based on Dialogflow's fulfillment text. If a session ID is not provided, the function should create one.
  
  assert detect_intent_text("What day is today?", "session_123")=="Today is Monday."
  assert detect_intent_text("What is the day after tomorrow?", "session_456")=="The day after tomorrow is Wednesday."
  assert detect_intent_text("Is today Tuesday?", "session_789")=="No, today is not Tuesday."
  assert detect_intent_text("What day was yesterday?", "session_012")=="Yesterday was Sunday."
  assert detect_intent_text("", "")=="Please provide a valid question."
output: def detect_intent_text(text, session_id=None):
      if not text:
          return "Please provide a valid question."
      if not session_id:
          session_id = str(uuid.uuid4())
      
      import dialogflow_v2 as dialogflow
      project_id = "your-project-id"
      session_client = dialogflow.SessionsClient()
      session = session_client.session_path(project_id, session_id)
      
      text_input = dialogflow.types.TextInput(text=text, language_code="en")
      query_input = dialogflow.types.QueryInput(text=text_input)
      
      response = session_client.detect_intent(session=session, query_input=query_input)
      
      return response.query_result.fulfillment_text