UI: Add bulk Clear on the Dag Runs list page (#67564)

Re-introduces collective Clear on the Dag Runs list page — the
Airflow 2.x ``DagRunModelView`` action that the Airflow 3 UI did not
yet replicate (#63854).

The button sits next to the bulk Delete shipped in #67095 and opens a
dialog mirroring the existing single-run Clear: a segmented control
(``Clear existing tasks`` / ``Clear only failed tasks`` / ``Queue up
new tasks``), an affected-tasks preview grouped by run, and an optional
note.

No backend change is required — the dialog fans out the existing
``POST /dags/{dag_id}/dagRuns/{dag_run_id}/clear`` endpoint over the
selected runs with ``Promise.allSettled``, then patches the note via
``PATCH /dags/{dag_id}/dagRuns/{dag_run_id}`` on the runs that
succeeded. Per-run outcomes are surfaced via the partial-failure UX
landed in #67284: successful rows are deselected, failures stay in the
selection and appear as inline errors so the user can retry just the
remaining set.

Bulk Mark as success / failed on Dag Runs (the other half of #63854)
is intentionally out of scope here.