Hide unreadable assets in the asset expression served by next_run_assets

[henry3260]

Why

DagModel.asset_expression names every upstream asset of a Dag, including their ids, names and uris. GET /ui/next_run_assets/{dag_id} returns it verbatim, so a caller with read access to a Dag can learn the identity of assets a fine-grained auth manager hides from them, even after the events list itself is scoped (#72862).

The expression is written by the Dag processor with no notion of a caller, and the scheduler needs it complete, so it cannot be filtered at write time. It has to be scoped per request, at the API layer, like the asset list endpoints already are.

This is the first of a small series: it introduces the shared redaction helper and the response-shape change, and wires the first endpoint. Follow-ups will wire the public GET /dags/{dag_id}/details, the UI GET /ui/dags list, and the UI rendering of a hidden leaf.

What

• airflow-core/src/airflow/api_fastapi/common/asset_expression.py: new redact_asset_expression helper. It walks the all / any tree and replaces every asset leaf whose id the caller may not read with {"uri": null, "name": null, "id": null, "group": ..., "hidden": true}. The slot is kept so the shape of the schedule stays honest instead of silently dropping a condition. A leaf without an id fails closed. alias and asset_ref leaves are left as-is (no batch authorization exists for aliases; refs are unresolved names by design). The input is never mutated, since it lives on an ORM row whose session commits on exit.
• airflow-core/src/airflow/api_fastapi/core_api/datamodels/common.py: AssetExpressionAssetInfo gains hidden: bool = False, and uri / name become nullable to carry a redacted leaf.
• airflow-core/src/airflow/api_fastapi/core_api/routes/ui/assets.py: next_run_assets injects ReadableAssetsFilterDep and serves the redacted expression on all three return paths.

---

Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Claude Fable 5.1)