Always provide a relevant TI context in Dag callback

[Asquator]

DRAFT

This PR strives to make sense out of TI contexts passed to Dag callbacks. Rather than always passing the last task lexicogrpahically, the new logic passes the last "relevant" task to Dag's failure.

• closes DAG on_failure_callback uses wrong context #26760

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

• Yes (please specify the tool below)
  deepwiki.com was used to find the relevant logic in the repo, and an embedded chat was used to make a skeleton for TestDagRunGetFirstTiCausingFailure.

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[Asquator]

I thought that tasks with null state should also be always marked as skipped (the in_ clause won't fetch them because of SQL semantics).