Fix/nickname preserve

[AJ-Collins]

Summary

This pull request fixes a bug where user-provided nickname values were being overridden by system-generated nicknames during registration.

What was happening

The current logic in User.create() assigns a new nickname using generate_nickname() regardless of whether the user supplied one or not.

As a result, even when a user provides a valid nickname in the registration payload, it gets replaced by a generated one (e.g., clever_fox_375), which is unexpected behavior.

Fixes

• Updated the logic to:
  • Check if the user provided a nickname.
  • Validate uniqueness if provided.
  • Generate one only if not supplied.
• Logs an error and aborts if a duplicate nickname is found.

Code changes

# Old logic (replaced)
new_nickname = generate_nickname()
while await cls.get_by_nickname(session, new_nickname):
    new_nickname = generate_nickname()
new_user.nickname = new_nickname

# New logic
nickname = validated_data.get('nickname')
if not nickname:
    nickname = generate_nickname()
    while await cls.get_by_nickname(session, nickname):
        nickname = generate_nickname()
else:
    if await cls.get_by_nickname(session, nickname):
        logger.error("Nickname already exists.")
        return None

new_user.nickname = nickname