chore: remove duplicated server_default from agent and credential permission models

[HandSonic]

Summary

For #29314 — a safe slice of the server_default cleanup.

• Remove duplicated server_default from 11 columns in api/models/agent.py and api/models/credential_permission.py that already declare an equivalent application-level default= (active_config_has_model, active_config_is_published, three status columns, draft_type, created_at with default=naive_utc_now, active_guard, value_owned_by_drive, is_skill, has_permission).
• server_default only affects migration autogeneration, not runtime behavior, and the duplication confuses new model definitions which copy these files as examples.
• No behavior change; no migration needed (existing database schemas are untouched by a model-definition-only change).

Intentionally left untouched: the two created_at columns (agent.py, credential_permission.py) that rely solely on server_default=func.current_timestamp() with no app-level default — removing those would change insert behavior and deserves its own decision (add an app-level default first, or keep the server default).

Testing

• uv run --project api pytest api/tests/unit_tests/models -q → 371 passed (2 failures in test_end_user_type.py are pre-existing on main and unrelated — verified on a clean tree)
• ruff check / ruff format --check on touched files → pass
• pyrefly check on touched files → 0 errors

From CodeBuddy Code