Fixed OpenAI Conersational Model

[MinuraPunchihewa]

Description

This PR achieves the following,

1. Fixes the conversational model by fixing the issue mentioned below.
2. Removes some code centered around the assistant_column that seems redundant. As shown here, this column no longer seems to be in the expected list of arguments. @paxcema Let me know what your thoughts are on this.

Fixes #9226

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)

Verification Process

To ensure the changes are working as expected:

• Test Location: Specify the URL or path for testing.
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Additional Media:

• I have attached a brief loom video or screenshots showcasing the new functionality or change.

conversational mode:

conversational-full mode:

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.

[paxcema]

Looks good, I think changes around invalid arguments make sense.