create_deep_linked_url accepts 4-character bot usernames, but Telegram requires a minimum of 5 #5284

Bug Description

create_deep_linked_url validates bot_username with len(bot_username) <= 3, which means 4-character usernames are accepted without error.

However, the Telegram Bot documentation explicitly states:

Usernames are 5-32 characters long and not case sensitive – but may only include Latin characters, numbers, and underscores.

Steps to Reproduce

from telegram.helpers import create_deep_linked_url

# Should raise ValueError (4 chars < 5 minimum) but doesn't:
url = create_deep_linked_url("xbot", "payload")  
print(url)  # https://t.me/xbot?start=payload  ← silently wrong

Expected behaviour

create_deep_linked_url("xbot", "payload") should raise ValueError because "xbot" is only 4 characters, below Telegram's 5-character minimum.

Actual behaviour

No error is raised; the function returns a URL with an invalid username.

Root Cause

In src/telegram/helpers.py the guard is:

if bot_username is None or len(bot_username) <= 3:
    raise ValueError("You must provide a valid bot_username.")

The condition should be len(bot_username) < 5 (i.e., <= 4) to match Telegram's actual minimum of 5 characters.

The docstring also incorrectly states "less than 4 characters" instead of "less than 5 characters".

Operating System

Any

Version of Python, python-telegram-bot & dependencies

Any

Relevant log output

N/A