search-with-proof: cluster-unavailable detection, kept messages, run-level abort

Zero running workers behind "no free workers available" now raises
ClusterUnavailable instead of waiting out the capacity deadline; CapacityDeadline
carries its message; cleanup failures no longer mask the original exception; the
pilot driver aborts the run on infrastructure failure before any model call.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

tests/test_search_task_storage.py

@@ -222,3 +222,21 @@ def test_scratch_failure_preserves_exit_and_stderr(workspace, monkeypatch):
     assert result["exit_code"] is not None
     assert result["stderr"] == "pip build detail"
     assert result["reason"].startswith("ScratchLimit:")
+
+
+def test_capacity_deadline_plain_diagnostic_survives():
+    from benchmarks.ax.batch import CapacityDeadline
+
+    message = "no free workers available before deadline"
+    assert task_storage.diagnostic(message) == message
+    assert task_storage.exception_reason(CapacityDeadline(message)) == "CapacityDeadline: " + message
+
+
+def test_cleanup_diagnostic_retains_both_causes():
+    error = ValueError("original apply failed")
+    error.executor_cleanup_reason = "executor cleanup failed: " + "details " * 100 + "delete failed"
+    error.args = (str(error) + "; " + error.executor_cleanup_reason,)
+    reason = task_storage.exception_reason(error)
+    assert reason.startswith("ValueError: original apply failed;")
+    assert reason.endswith("delete failed")
+    assert len(reason) <= 300