🐛 fix: Avatar Type Definitions in Agent/Assistant Schemas

[danny-avila]

Summary

Closes #6234 as introduced by #6210

• Updated agent schema by removing the specific object type for avatar and using Schema.Types.Mixed.
• Updated assistant schema similarly to replace the explicit avatar definition.
• Ensured that the default behavior for avatar remains unchanged.
• Validated schema consistency and passed local unit tests.

Change Type:

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

Testing:

• Ran local unit tests to verify schema validation and data consistency.
• Manually checked avatar field behavior in LibreChat to ensure compatibility.

Checklist:

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective
• Local unit tests pass with my changes