refactor: select in datasource_provider_service

[RenzoMXD]

Summary

• Migrate all 6 db.session.query() calls to SQLAlchemy 2.x select() style in datasource_provider_service.py
• Replace .filter_by().count() with db.session.scalar(select(func.count()).where())
• Replace .filter_by().first() with db.session.scalar(select().where().limit(1))
• Replace .where().all() with list(db.session.scalars(select().where()).all())
• Add default scalar/scalars return values to test fixture for compatibility
• Update 10 test mocks for affected methods only; other tests unchanged

Test plan

• All 63 unit tests pass for datasource_provider_service
• Basedpyright type check passes (0 errors)

Part of #22668

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[RenzoMXD]

@asukaminato0721
I think this is the end for the @22668.
Pleas let me know if there is another issue for help.
Thanks.