fix(agent): close tool-call sequence on all interrupt aborts, not just finalize_turn

#48879 closed the tool-call sequence on interrupt inside finalize_turn so a
/stop after a tool no longer persists a `tool` tail that the next user message
turns into a `tool -> user` role-alternation violation (which strict providers
like Gemini/Claude react to by hallucinating a continuation and ignoring prior
context — what users see as "lost context after stop").

But the retry-wait, error-handling, and post-error retry-wait interrupt aborts
in conversation_loop return early and never reach finalize_turn, so they still
persisted and returned a raw `tool` tail. Interrupting during provider
backoff/rate-limiting (common under heavy work) hit exactly this path.

Extract the close into a shared close_interrupted_tool_sequence helper and apply
it at every interrupt abort (finalize_turn + the three early returns) so the
whole bug class is fixed, not just the one site.

tests/agent/test_close_interrupted_tool_sequence.py

@@ -0,0 +1,86 @@
+"""Regression tests for ``close_interrupted_tool_sequence`` (#48879 follow-up).
+
+#48879 closed the tool-call sequence on interrupt inside ``finalize_turn``,
+but the retry/backoff/error interrupt aborts in ``conversation_loop`` ``return``
+early and never reach it — so they persisted a raw ``tool`` tail. The next user
+message then lands as ``... tool → user``, the role-alternation violation that
+makes strict providers (Gemini, Claude) hallucinate a continuation and ignore
+prior context (what the user perceives as "lost context").
+
+The fix routes every interrupt abort through this one shared helper. These tests
+pin the helper's contract and prove the post-interrupt + next-user-message
+transcript is alternation-safe.
+"""
+
+from agent.message_sanitization import close_interrupted_tool_sequence
+
+
+def _tool_tail():
+    return [
+        {"role": "user", "content": "edit the file"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "patch", "arguments": "{}"}}],
+        },
+        {"role": "tool", "tool_call_id": "c1", "content": "ok edited"},
+    ]
+
+
+def _assert_no_tool_then_user(messages):
+    for i in range(len(messages) - 1):
+        if messages[i].get("role") == "tool":
+            assert messages[i + 1].get("role") != "user", (
+                f"role-alternation violation: tool → user at index {i}"
+            )
+
+
+def test_tool_tail_is_closed_with_placeholder():
+    messages = _tool_tail()
+    assert close_interrupted_tool_sequence(messages, None) is True
+    assert messages[-1]["role"] == "assistant"
+    assert messages[-1]["content"] == "Operation interrupted."
+
+
+def test_tool_tail_keeps_interrupt_text_when_present():
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, "Operation interrupted during retry (attempt 2/3).")
+    assert messages[-1]["role"] == "assistant"
+    assert messages[-1]["content"] == "Operation interrupted during retry (attempt 2/3)."
+
+
+def test_blank_interrupt_text_falls_back_to_placeholder():
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, "   ")
+    assert messages[-1]["content"] == "Operation interrupted."
+
+
+def test_closing_makes_next_user_message_alternation_safe():
+    """The whole point: appending a user turn after the close must not
+    produce the ``tool → user`` shape strict providers choke on."""
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, None)
+    follow_on = messages + [{"role": "user", "content": "they do! increase the timing"}]
+    _assert_no_tool_then_user(follow_on)
+
+
+def test_assistant_tail_is_left_untouched():
+    messages = [
+        {"role": "user", "content": "hi"},
+        {"role": "assistant", "content": "partial reply"},
+    ]
+    before = [dict(m) for m in messages]
+    assert close_interrupted_tool_sequence(messages, "interrupted") is False
+    assert messages == before
+
+
+def test_user_tail_is_left_untouched():
+    messages = [{"role": "user", "content": "hi"}]
+    assert close_interrupted_tool_sequence(messages, None) is False
+    assert len(messages) == 1
+
+
+def test_empty_messages_is_noop():
+    messages = []
+    assert close_interrupted_tool_sequence(messages, "x") is False
+    assert messages == []