fix(ansible): runs end cleanly: output read in chunks, the process group always ended, a cancelled task recorded (#5156)

- Output is read in 64 KiB chunks and split into lines here; a line past
  64 KiB is cut and marked instead of raising out of readline's limit.
- ansible-playbook runs in its own session; whatever ends the read (finish,
  an error, the task being cancelled) SIGTERMs then SIGKILLs its whole group,
  so no ssh or worker it forked outlives the run.
- A run whose task is cancelled (shutdown) records 'cancelled' with
  finished_at before re-raising, instead of staying 'running'.
- A cancel that lands after the process exited 0 no longer turns a
  successful run into 'cancelled'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_ansible_executor.py

@@ -28,6 +28,34 @@ def test_parse_recap_empty_without_recap():
     assert parse_recap(["TASK [x] ***", "ok: [h]"]) == {}
 
 
+class _Stream:
+    def __init__(self, chunks):
+        self.chunks = list(chunks)
+
+    async def read(self, _n):
+        return self.chunks.pop(0) if self.chunks else b""
+
+
+def _read_lines(chunks, monkeypatch, limit=8):
+    import asyncio
+    from steward.ansible import executor
+    monkeypatch.setattr(executor, "_MAX_LINE_BYTES", limit)
+
+    async def _go():
+        return [line async for line in executor._lines(_Stream(chunks))]
+    return asyncio.run(_go())
+
+
+def test_lines_split_across_chunks(monkeypatch):
+    assert _read_lines([b"ab", b"c\nde", b"f\n", b"tail"], monkeypatch) == [b"abc", b"def", b"tail"]
+
+
+def test_a_line_past_the_limit_is_cut_and_the_rest_dropped(monkeypatch):
+    # #5156: a long line can't stall or break the reader; the next line is intact.
+    got = _read_lines([b"0123456789", b"abcdef", b"gh\nnext\n"], monkeypatch)
+    assert got == [b"0123456789"[:8] + b" [line cut at 8 bytes]", b"next"]
+
+
 def test_cancel_run_flags_untracked_run():
     rid = "nonexistent-run-id"
     assert cancel_run(rid) is True