Widen type hint for DagRun.get_task_instances / fetch_task_instances state parameter

[tysoncung]

Fixes #67653

The state parameter on DagRun.get_task_instances and DagRun.fetch_task_instances was annotated as Iterable[TaskInstanceState | None] | None, but the runtime handles a single TaskInstanceState value via an isinstance(state, str) short-circuit. Since TaskInstanceState(str, Enum) inherits from str, calling with state=TaskInstanceState.FAILED works at runtime but mypy/pyright flags it.

Fix: Widen the annotation to TaskInstanceState | Iterable[TaskInstanceState | None] | None on both methods to match the actual runtime behaviour.

[github-actions]

Hi maintainer, this PR was merged without a milestone set.
We've automatically set the milestone to Airflow 3.2.3 based on: backport label targeting v3-2-test
If this milestone is not correct, please update it to the appropriate milestone.

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[github-actions]

Backport failed to create: v3-2-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-2-test

You can attempt to backport this manually by running:

cherry_picker fb56cef v3-2-test

This should apply the commit to the v3-2-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.