Fix DagRun.get_task_instances state type hint to accept a single state

[bmaquet]

Summary

• DagRun.get_task_instances and DagRun.fetch_task_instances already accept a single TaskInstanceState at runtime via the isinstance(state, str) branch (Airflow's state enums inherit from str), but the type hint only declares Iterable[TaskInstanceState | None] | None.
• Widens the annotation to TaskInstanceState | Iterable[TaskInstanceState | None] | None to match existing runtime behavior. No behavior change.
• Adds unit test in test_dagrun.py to validate get_task_instances accepts either an iterable or a single instance of TaskInstanceState

Test Plan

• prek run mypy-airflow-core --files airflow-core/src/airflow/models/dagrun.py
• uv run --project airflow-core pytest airflow-core/tests/unit/models/test_dagrun.py -xvs

closes: #67653

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  claude

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[boring-cyborg]

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[jscheffl]

Looks reasonable as the logic was handling. As it is an interface/public API change would like to have another pair of eyes on this but see no harm in merging.

[bmaquet]

It seems another PR to fix this issue was created after mine (#67880) and was merged before this one. I believe we can close this one then.