core/agent: keep executed tool results when a failed turn is rolled back

A tool batch counted as committed only once every call had its result, so a
batch cut short was dropped whole and the retry ran tools that had already
had effects. Keep the calls that did report a result: their assistant row
stays as the pairing carrier (a request must answer every tool_calls id),
without the text of the unfinished turn, and the calls without a result are
absent so the retry re-issues them instead of assuming they ran. The kept
rows go to the store right away, and a failed rollback write is not allowed
to mask the failure being rolled back.

tests/test_agent/test_rollback.py

@@ -0,0 +1,99 @@
+"""Rollback of a failed turn keeps the tool work that already ran.
+
+``committed_prefix_end`` commits a batch only once every call has its result, so
+``rollback_tail`` keeps the executed part of a batch that was cut short: the
+retry then continues from real results instead of running those tools again.
+"""
+
+from __future__ import annotations
+
+from msgspec import UNSET
+
+from plyngent.agent.chat import rollback_tail
+from plyngent.lmproto.openai_compatible.model import (
+    AnyChatMessage,
+    AssistantChatMessage,
+    AssistantFunctionTool,
+    AssistantFunctionToolCall,
+    DeveloperChatMessage,
+    ToolChatMessage,
+    UserChatMessage,
+)
+
+
+def _call(call_id: str, name: str = "some_tool") -> AssistantFunctionToolCall:
+    return AssistantFunctionToolCall(id=call_id, function=AssistantFunctionTool(name=name, arguments="{}"))
+
+
+def test_rollback_tail_keeps_the_executed_call_of_a_cut_short_batch() -> None:
+    messages: list[AnyChatMessage] = [
+        UserChatMessage(content="do both"),
+        AssistantChatMessage(content="first, then the other", tool_calls=[_call("call_a"), _call("call_b")]),
+        ToolChatMessage(tool_call_id="call_a", content="a ran"),
+    ]
+
+    tail = rollback_tail(messages, 1)
+
+    assert len(tail) == 2
+    record, result = tail
+    assert isinstance(record, AssistantChatMessage)
+    # The assistant row survives only as the carrier of the pairing.
+    assert record.content is UNSET
+    assert record.reasoning_content is UNSET
+    calls = record.tool_calls
+    assert calls is not UNSET
+    assert [call.id for call in calls] == ["call_a"]
+    assert isinstance(result, ToolChatMessage)
+    assert result.content == "a ran"
+
+
+def test_rollback_tail_drops_a_batch_that_reported_nothing() -> None:
+    messages: list[AnyChatMessage] = [
+        UserChatMessage(content="go"),
+        AssistantChatMessage(content="about to call", tool_calls=[_call("call_a")]),
+    ]
+
+    assert rollback_tail(messages, 1) == []
+
+
+def test_rollback_tail_drops_unfinished_text_and_keeps_developer_rows() -> None:
+    messages: list[AnyChatMessage] = [
+        UserChatMessage(content="go"),
+        AssistantChatMessage(content=UNSET, tool_calls=[_call("call_a")]),
+        ToolChatMessage(tool_call_id="call_a", content="a ran"),
+        DeveloperChatMessage(content="checkpoint"),
+        AssistantChatMessage(content="half-written answer"),
+    ]
+
+    tail = rollback_tail(messages, 1)
+
+    assert [type(message) for message in tail] == [AssistantChatMessage, ToolChatMessage, DeveloperChatMessage]
+
+
+def test_rollback_tail_pairs_a_repeated_call_id_once() -> None:
+    messages: list[AnyChatMessage] = [
+        UserChatMessage(content="go"),
+        AssistantChatMessage(content=UNSET, tool_calls=[_call("call_a"), _call("call_a")]),
+        ToolChatMessage(tool_call_id="call_a", content="only once"),
+    ]
+
+    tail = rollback_tail(messages, 1)
+
+    assert len(tail) == 2
+    record, result = tail
+    assert isinstance(record, AssistantChatMessage)
+    calls = record.tool_calls
+    assert calls is not UNSET
+    assert [call.id for call in calls] == ["call_a"]
+    assert isinstance(result, ToolChatMessage)
+    assert result.content == "only once"
+
+
+def test_rollback_tail_ignores_rows_before_start() -> None:
+    messages: list[AnyChatMessage] = [
+        UserChatMessage(content="go"),
+        AssistantChatMessage(content=UNSET, tool_calls=[_call("call_a")]),
+        ToolChatMessage(tool_call_id="call_a", content="a ran"),
+    ]
+
+    assert rollback_tail(messages, len(messages)) == []