fix(task-5): a rollback that fails after a cancellation no longer kills the session, and the COM rollback goes back to an UNDO Mark instead of prompting

Two review findings on 32821c6.

1. Once the shielded rollback body runs after a real client cancellation it
   can fail (the COM hard timeout, _safe_send_command's own deadline, an
   ezdxf quarantine refusal). That RuntimeError escaped run_spec as an
   ordinary error and the mcp lowlevel server, which had already answered
   the cancelled request, tried to respond a second time: `assert not
   self._completed` in mcp/shared/session.py escaped the server task group
   and the whole session died (measured through the real transport: the next
   system_status was never answered and Client.__aexit__ re-raised the
   AssertionError). run_spec now decides by the exception it is handling:
   after a cancellation the rollback error is logged and the cancellation is
   what propagates, so the SDK's cancellation suppression applies; after any
   other error the rollback error propagates as before, with the original
   chained as its context.

2. The docstring promised the rollback on both engines, but on the live
   AutoCAD 2026 ComBackend.transaction_rollback sent `_UNDO B` with no UNDO
   Mark in the stack (StartUndoMark/EndUndoMark form an undo *group*, not a
   Mark), AutoCAD answered "This will undo everything. OK? <Y>", SendCommand
   blocked at that prompt, _run gave up after 60 s, the entities stayed and
   every COM client was rejected until a human pressed ESC. transaction_begin
   now types `_.UNDO _M` before StartUndoMark (refused with ok:false while
   CMDACTIVE, where the Mark would be swallowed as input), and
   transaction_rollback ends the group and sends `_.UNDO _B`, which returns
   exactly to that Mark - also on an empty transaction, where `_UNDO 1` would
   be one step too many. A rollback with no transaction is refused the way the
   headless engine refuses it, because an unmarked Back is that prompt.
   Measured with the real ComBackend in a scratch Documents.Add() document on
   the running AutoCAD 2026 (closed with Close(False)): begin 31 ms ->
   entity_create_line + circle -> rollback 63 ms -> 3 entities back to 1,
   CMDACTIVE 0, no prompt; an empty begin/rollback leaves the older line;
   begin/commit unchanged.

Tests: the two failure modes on run_spec (anyio scope cancellation with a
raising rollback completes the task group and logs; a ValueError with a
raising rollback still raises the rollback error with the ValueError as
context), the real-transport case (client.cancel mid-run, rollback raises,
the next request is answered, cleanup through the real rollback), the COM
shape now pins `_.UNDO _M, StartUndoMark, EndUndoMark, _.UNDO _B`, and
test_com_sprint3 grows a Mark-keeping fake document whose unmarked Back
fails the way the prompt does: begin before the group, rollback consumes the
Mark, rollback without a transaction sends nothing, begin refuses on
CMDACTIVE.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

tests/test_com_sprint3.py

@@ -101,7 +101,7 @@ async def test_run_lisp_captures_value_from_users1(monkeypatch):
 
 async def test_transaction_begin_sets_flag_and_marks_undo(monkeypatch):
     doc = _Doc()
-    doc.StartUndoMark = MagicMock()
+    doc.StartUndoMark = MagicMock(side_effect=lambda: doc.sent.append("StartUndoMark"))
     monkeypatch.setattr(cb, "_acad_doc", lambda: doc)
 
     b = _backend_no_executor()
@@ -109,6 +109,25 @@ async def test_transaction_begin_sets_flag_and_marks_undo(monkeypatch):
     assert out["ok"] is True
     assert b._transaction_active is True
     doc.StartUndoMark.assert_called_once()
+    # The UNDO Mark goes in *before* the undo group: `_UNDO _B` in the rollback
+    # returns to it, and a Mark inside the group would be undone with it.
+    assert doc.sent == ["_.UNDO _M\n", "StartUndoMark"], doc.sent
+
+
+async def test_transaction_begin_refuses_while_a_prompt_is_active(monkeypatch):
+    """With CMDACTIVE set, SendCommand would feed `_UNDO _M` to the active
+    command as input and no Mark would exist for the rollback to reach."""
+    doc = _Doc()
+    doc.SetVariable("CMDACTIVE", 1)
+    doc.StartUndoMark = MagicMock()
+    monkeypatch.setattr(cb, "_acad_doc", lambda: doc)
+
+    b = _backend_no_executor()
+    out = await b.transaction_begin()
+    assert out["ok"] is False and "CMDACTIVE" in out["error"] and "ESC" in out["error"]
+    assert b._transaction_active is False, "nothing reached AutoCAD"
+    assert doc.sent == []
+    doc.StartUndoMark.assert_not_called()
 
 
 async def test_transaction_commit_clears_flag_on_error():
@@ -124,17 +143,63 @@ async def test_transaction_commit_clears_flag_on_error():
     assert b._transaction_active is False  # not left stale
 
 
-async def test_transaction_rollback_uses_safe_send_command(monkeypatch):
-    doc = _Doc()  # idle: CMDACTIVE=0 so _safe_send_command returns promptly
+class _MarkingDoc(_Doc):
+    """An idle doc that keeps AutoCAD's UNDO Mark bookkeeping.
+
+    On the live AutoCAD 2026 `_UNDO B` with no Mark answers "This will undo
+    everything. OK? <Y>" and `SendCommand` blocks at that prompt until a human
+    presses ESC (measured: the rollback hit the 60 s COM timeout, the entities
+    stayed, every COM client was rejected meanwhile). `StartUndoMark` /
+    `EndUndoMark` form an undo *group*, not a Mark. This fake answers the
+    unmarked Back the way the prompt would end up: as a failure, not a hang.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self.marks = 0
+        self.group_open = False
+
+    def SendCommand(self, cmd):
+        super().SendCommand(cmd)
+        if cmd.strip() == "_.UNDO _M":
+            self.marks += 1
+        elif cmd.strip() == "_.UNDO _B":
+            if self.marks == 0:
+                raise RuntimeError("prompt: This will undo everything. OK? <Y>")
+            self.marks -= 1
+
+    def StartUndoMark(self):
+        self.group_open = True
+
+    def EndUndoMark(self):
+        self.group_open = False
+
+
+async def test_transaction_rollback_goes_back_to_the_mark_begin_set(monkeypatch):
+    doc = _MarkingDoc()  # idle: CMDACTIVE=0 so _safe_send_command returns promptly
     monkeypatch.setattr(cb, "_acad_doc", lambda: doc)
 
     b = _backend_no_executor()
-    b._transaction_active = True
+    assert (await b.transaction_begin())["ok"] is True
     out = await b.transaction_rollback()
     assert out["ok"] is True
     assert b._transaction_active is False
-    # UNDO B went through _safe_send_command (which appends a trailing newline).
-    assert any("_UNDO B" in c for c in doc.sent)
+    # UNDO Back went through _safe_send_command (which appends a trailing
+    # newline) and found the Mark begin set: no prompt, group closed, Mark consumed.
+    assert [c.strip() for c in doc.sent] == ["_.UNDO _M", "_.UNDO _B"]
+    assert doc.marks == 0 and doc.group_open is False
+
+
+async def test_transaction_rollback_without_a_transaction_is_refused(monkeypatch):
+    """No transaction means no Mark: an unmarked `_UNDO _B` is the prompt
+    that strands the STA worker, so it is never sent."""
+    doc = _MarkingDoc()
+    monkeypatch.setattr(cb, "_acad_doc", lambda: doc)
+
+    b = _backend_no_executor()
+    out = await b.transaction_rollback()
+    assert out == {"ok": False, "error": "No active transaction to rollback"}
+    assert doc.sent == []
 
 
 # ── R13 — offset honors side_x/side_y and deletes the unused copy ───────────