Eager-load DagCode in the Dag source API #73716

Description

GET /dags/{dag_id}/dagSources loads the DagVersion first. The dag_code
relationship is lazy, so reading dag_version.dag_code sends a second query.
Loading dag_code together with the version (e.g. via joinedload) would
avoid that extra database round trip.

Use case/motivation

One query instead of two for each Dag source request. The change affects both
the model layer (DagVersion.get_version) and the API route, so it needs its
own PR.

Related issues

Follow-up from review of #72474.

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct