Replace CTE with subquery in asset orphanage to support MySQL backends without CTE-in-DML

[saitejabandaru-in]

Closes #72187.

Motivation

MySQL-compatible backends like Vitess/PlanetScale do not support CTEs in DML statements. When SchedulerJobRunner._update_asset_orphanage runs its asset reference-count query as a CTE and executes a DELETE, the query planner rejects it. Because this runs on the parsing cleanup interval, it causes the scheduler to crash-loop.

Changes

• Replaced .cte() with .subquery() for both orphan_query and activate_query in _update_asset_orphanage.
• Updated the type hints for assets_query in _orphan_unreferenced_assets and _activate_referenced_assets from CTE to Subquery.

Since each query is consumed by a single statement, the CTE is not semantically required and a standard derived-table subquery achieves the same outcome. This unblocks deployments on Vitess without changing query semantics for Postgres or standard MySQL.

[SameerMesiah97]

1. This needs a newsfragment filed under 'bugfix'
2. Could we add a regression test that compiles the resulting DML statements using the MySQL dialect and verifies they do not contain a WITH clause? Otherwise, tests running against other backends may not catch a regression to the unsupported CTE-in-DML form.

[GyuhoonK]

Hi @saitejabandaru-in, thanks for looking into this! Just a heads-up that this issue is already being addressed by #72188, which was opened earlier for the same issue (#72187) and takes the same approach (replacing .cte() with .subquery()). To avoid duplicate effort, it would probably make sense to consolidate the review on the earlier PR.