[🐛 BUG] var_name renders as empty string in chat's on_action's callback

[enarroied]

🔍 What went wrong?

When using the chat visual element, I can trigger a callback, it's the on_action parameter.
According to the documentation, the callback's signature has a var_name argument in the second position, and it references the name of the messages variable. However, it returns an empty string instead.

✅ Expected Behavior

It should return the name of the variable :)

🔄 Steps to Reproduce

The following code reproduces the issue, just send messages and check the terminal, the strings for var_name are empty, but the variable has a name (it can't not have it):

from taipy.gui import Gui
from taipy.gui import builder as tgb


def say_hi():
    return "hi"


def chat(state, var_name: str, payload: dict):
    # Retrieve the callback parameters
    print(var_name)  # Prints nothing (empty line)
    print(len(var_name))  # It's has 0 length, it's coherent with the empty line
    print(type(var_name))  # It's a string indeed

    (_, _, user_message, sender_id) = payload.get("args", [])
    # Add the input content as a sent message
    messages.append((f"{len(messages)}", user_message, sender_id))

    try:
        result_message = say_hi()
    except Exception:
        result_message = "There was a problem!"
    messages.append((f"{len(messages)}", result_message, users[1]))
    state.messages = messages


with tgb.Page() as chat_page:
    tgb.text("# PII Analyzer", mode="md")
    tgb.html("hr")
    tgb.chat(
        messages="{messages}",
        users="{users}",
        show_sender=True,
        sender_id="{users[0]}",
        on_action=chat,
        mode="markdown",
    )

if __name__ == "__main__":
    users = ["User", "Bot"]

    messages = []  # This is the message variable, its name is "messages"
    gui = Gui(chat_page)
    gui.run(title="var_name not showing", dark_mode=False, use_reloader=True)

💻 Runtime Environment

Lubuntu 24.04

🌐 Browser (if applicable)

Brave

🖥️ Operating System

Linux

📦 Taipy Version

4.0.2 and 4.0.3

📋 Additional Context (Optional)

📜 Code of Conduct

• I have checked the existing issues to avoid duplicates.
• I am willing to work on this issue (optional)

✅ Acceptance Criteria

• A reproducible unit test is added.
• Code coverage is at least 90%.
• The bug reporter validated the fix.
• Relevant documentation updates or an issue created in