[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

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Additional Context

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