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_timeout.py

@@ -167,6 +167,12 @@ async def test_timed_out_transaction_begin_does_not_hide_an_open_undo_mark(
     marks = []
 
     class _FakeDoc:
+        def GetVariable(self, name):
+            return 0  # CMDACTIVE clear
+
+        def SendCommand(self, cmd):
+            marks.append(cmd.strip())  # the UNDO Mark begin sets first
+
         def StartUndoMark(self):
             marks.append("start")
             try:
@@ -185,7 +191,7 @@ async def test_timed_out_transaction_begin_does_not_hide_an_open_undo_mark(
         )
         again = await com_backend.transaction_begin()
         assert again == {"ok": False, "error": "A transaction is already active"}
-        assert marks == ["start"], "the second begin must not nest another undo mark"
+        assert marks == ["_.UNDO _M", "start"], "the second begin must not nest another undo mark"
     finally:
         release.set()
         assert finished.wait(5)