Added defensive "before" validator in DagRunContext to handle DetachedInstanceError

[SameerMesiah97]

Description

A defensive (mode = "before") Pydantic validator has been added to DagRunContext to safely handle detached ORM DagRun objects. The validator reloads the DagRun object from the DB with consumed_asset_events when relationship access would otherwise raise DetachedInstanceError, preventing scheduler crashes in DagRun timeout and callback paths. This complements existing eager-loading fixes added in PR #56916 and PR #59714.

Rationale

DagRunContext may be constructed with ORM DagRun instances that are no longer session-bound (e.g. during scheduler timeout handling). Accessing lazily loaded relationships such as consumed_asset_events on detached instances can raise DetachedInstanceError, which currently propagates and causes the scheduler to exit unexpectedly.

This issue has surfaced multiple times in different DagRun code paths. Centralizing a defensive guard in DagRunContext ensures that missing or detached relationship data does not crash the scheduler. Using a session to reload the DagRun within the validator is an acceptable tradeoff to maintain correctness with regards to the state of consumed_asset_events. Also, it is assumed that the reloaded DagRun entity must exist in the DB for code paths where DagRunContext is called.

Note that this validator only applies to ORM DagRun objects and not fastapi data models.

Tests

• Added a test covering DagRunContext construction with a detached ORM DagRun, asserting that consumed_asset_events is present afterwards.
• Added a test covering the attached ORM DagRun path to ensure existing behavior is preserved.
• Existing DagRunContext tests for non-ORM/datamodel inputs remain unchanged and continue to pass.

Closes: #59740

[SameerMesiah97]

@Lee-W @uranusjr

Sorry for tagging you here. But I just wanted to know if any more action is required from me on this.

Also, I just wanted to know if this is ready for merge or if it requires further review.

[Lee-W]

Nothing is needed on your end. I just want to wait for a second pair of eyes :) let's wait for a bit longer.