[Chore/Refactor] Remove `server_default` in model definitions

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### Description

## Summary

PR #28188 introduced MySQL support and adjusted migration accordingly. However, the model definition is not adjusted, and most models still contains the `server_default=sa.text('uuid_generate_v4()')` option. While the `server_default` argument is only used to generate migration and not applied while running application, it still causes confusion, especially for new model definitions, as implementors of new models (both real humans and AIs) generally references existing model definitions as examples.

## Solution

Remove the `server_default` parameter from UUID column definitions in model files, allowing the application layer to handle database-specific UUID generation by setting the `default=uuid.uuid4` option for those models.

### Motivation

_No response_

### Additional Context

_No response_