revert PREFIX of fastapi server.

[shkangomelet]

After the changes introduced in this pull request, issue #1904 occurred, leading to a rollback of the changes.

I tested running the following script locally using the command:

chainlit run demo.py -w --root-path /app

demo.py

import chainlit as cl


@cl.step(type="tool")
async def tool():
    # Fake tool
    await cl.sleep(2)
    return "Response from the tool!"


@cl.on_message  # this function will be called every time a user inputs a message in the UI
async def main(message: cl.Message):
    """
    This function is called every time a user inputs a message in the UI.
    It sends back an intermediate response from the tool, followed by the final answer.

    Args:
        message: The user's message.

    Returns:
        None.
    """


    # Call the tool
    tool_res = await tool()

    await cl.Message(content=tool_res).send()

The script runs without any issues in this setup. However, I have not yet investigated any potential side effects. I would appreciate it if you could review this further.