feat: regenerate registers stream task in _in_flight_tasks (T83.1)

Both the primary and the interjection sub-stream in
``regenerate_assistant_turn`` are now wrapped in ``asyncio.create_task``
and registered in the chat-keyed ``_in_flight_tasks`` registry that the
``/turns/cancel`` route reads. Without this, hitting Stop during a
mid-regenerate stream was a no-op.

Mirrors the meanwhile registration pattern in chat/web/meanwhile.py
(snapshot-tested by tests/test_meanwhile_turn_flow.py).

Test added: test_regenerate_registers_task_in_in_flight_tasks captures
``"chat_bot_a" in _in_flight_tasks`` at the first stream yield via a
custom MockLLMClient subclass and asserts post-flight cleanup.

chat/services/regenerate.py

@@ -68,6 +68,7 @@ Phase 2.5 changes:
 
 from __future__ import annotations
 
+import asyncio
 import json
 from sqlite3 import Connection
 
@@ -250,19 +251,37 @@ async def regenerate_assistant_turn(
         guest_id=guest_bot_id,
     )
 
-    # 5. Stream the new narrative.
+    # 5. Stream the new narrative. T83.1: register the streaming Task in
+    # the chat-keyed in-flight registry so POST /chats/<id>/turns/cancel
+    # can call ``.cancel()`` on a mid-regenerate stream. We import the
+    # underscore name from turns.py deliberately — same single-process
+    # registry the cancel route reads, mirrors the meanwhile registration
+    # pattern in chat/web/meanwhile.py.
+    from chat.web.turns import _in_flight_tasks  # noqa: PLC0415
+
     accumulated: list[str] = []
-    async for chunk in client.stream(
-        messages,
-        model=settings.narrative_model,
-        max_tokens=settings.narrative_max_tokens,
-        temperature=settings.narrative_temperature,
-    ):
-        accumulated.append(chunk)
-        await publish(
-            chat_id,
-            {"event": "token", "text": chunk, "speaker_id": speaker_bot_id},
-        )
+
+    async def _stream_primary() -> None:
+        async for chunk in client.stream(
+            messages,
+            model=settings.narrative_model,
+            max_tokens=settings.narrative_max_tokens,
+            temperature=settings.narrative_temperature,
+        ):
+            accumulated.append(chunk)
+            await publish(
+                chat_id,
+                {"event": "token", "text": chunk, "speaker_id": speaker_bot_id},
+            )
+
+    stream_task = asyncio.create_task(_stream_primary())
+    _in_flight_tasks[chat_id] = stream_task
+    try:
+        await stream_task
+    finally:
+        # Always unregister so a subsequent turn / regenerate can register
+        # a fresh task. Mirrors the cleanup in turns.py::post_turn.
+        _in_flight_tasks.pop(chat_id, None)
     new_text = "".join(accumulated)
 
     # 6. Append the new assistant_turn event. ``user_turn_id`` points at
@@ -497,21 +516,32 @@ async def regenerate_assistant_turn(
             )
 
             interject_accumulated: list[str] = []
-            async for chunk in client.stream(
-                interject_messages,
-                model=settings.narrative_model,
-                max_tokens=settings.narrative_max_tokens,
-                temperature=settings.narrative_temperature,
-            ):
-                interject_accumulated.append(chunk)
-                await publish(
-                    chat_id,
-                    {
-                        "event": "token",
-                        "text": chunk,
-                        "speaker_id": silent_witness_id,
-                    },
-                )
+
+            async def _stream_interjection() -> None:
+                async for chunk in client.stream(
+                    interject_messages,
+                    model=settings.narrative_model,
+                    max_tokens=settings.narrative_max_tokens,
+                    temperature=settings.narrative_temperature,
+                ):
+                    interject_accumulated.append(chunk)
+                    await publish(
+                        chat_id,
+                        {
+                            "event": "token",
+                            "text": chunk,
+                            "speaker_id": silent_witness_id,
+                        },
+                    )
+
+            # T83.1: register the interjection sub-stream in the same
+            # in-flight registry so /turns/cancel collapses it too.
+            interject_task = asyncio.create_task(_stream_interjection())
+            _in_flight_tasks[chat_id] = interject_task
+            try:
+                await interject_task
+            finally:
+                _in_flight_tasks.pop(chat_id, None)
             interject_text = "".join(interject_accumulated)
 
             new_interjection_event_id = append_event(

tests/test_regenerate.py

@@ -662,3 +662,85 @@ def test_regenerate_drops_interjection_when_classifier_returns_false(
         new_primary_payload = json.loads(cur[0][0])
         assert new_primary_payload["text"] == "New primary text."
         assert "interjection_of" not in new_primary_payload
+
+
+def test_regenerate_registers_task_in_in_flight_tasks(tmp_path, monkeypatch):
+    """T83.1: regenerate's streaming Task is registered in the chat-keyed
+    ``_in_flight_tasks`` dict so the /turns/cancel route can cancel a
+    mid-regenerate stream. Mirrors the meanwhile registration pattern
+    pinned by tests/test_meanwhile_turn_flow.py.
+
+    Snapshot pattern: a custom MockLLMClient subclass captures the
+    presence of the chat_id in ``_in_flight_tasks`` at the first stream
+    yield (when the regenerate coroutine is awaiting our generator and
+    the task is alive). Post-flight, the entry must be cleaned up so the
+    next regenerate / turn registers a fresh task.
+    """
+    import asyncio
+    from typing import AsyncIterator, Sequence
+
+    from chat.config import Settings
+    from chat.db.migrate import apply_migrations
+    from chat.llm.client import Message
+    from chat.services.regenerate import regenerate_assistant_turn
+    from chat.web.turns import _in_flight_tasks
+
+    db_path = tmp_path / "test.db"
+    cfg = tmp_path / "config.toml"
+    cfg.write_text('featherless_api_key = "test"\n')
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(cfg))
+    monkeypatch.setenv("CHAT_DB_PATH", str(db_path))
+    apply_migrations(db_path)
+
+    _ut_id, at_id = _seed_with_one_turn(db_path)
+
+    in_flight_snapshot: dict = {}
+
+    class _SnapshotMock(MockLLMClient):
+        async def stream(
+            self, messages: Sequence[Message], *, model: str, **params
+        ) -> AsyncIterator[str]:
+            text = self._canned.pop(0)
+            for i, ch in enumerate(text):
+                if i == 0:
+                    in_flight_snapshot["present"] = (
+                        "chat_bot_a" in _in_flight_tasks
+                    )
+                    in_flight_snapshot["task"] = _in_flight_tasks.get(
+                        "chat_bot_a"
+                    )
+                yield ch
+
+    state_canned = json.dumps(
+        {"affinity_delta": 0, "trust_delta": 0, "knowledge_facts": []}
+    )
+    mock_client = _SnapshotMock(
+        canned=["Refreshed reply.", state_canned, state_canned]
+    )
+
+    settings = Settings(featherless_api_key="test")
+
+    # Pre-condition: registry empty for this chat.
+    assert "chat_bot_a" not in _in_flight_tasks
+
+    with open_db(db_path) as conn:
+        new_text = asyncio.run(
+            regenerate_assistant_turn(
+                conn,
+                mock_client,
+                settings=settings,
+                chat_id="chat_bot_a",
+                original_assistant_event_id=at_id,
+            )
+        )
+        assert new_text == "Refreshed reply."
+
+    # Mid-flight: the streaming task was present in the registry, and
+    # the captured value was an asyncio.Task.
+    assert in_flight_snapshot.get("present") is True, (
+        "_in_flight_tasks was empty at first yield — regenerate stream "
+        "isn't registering its task"
+    )
+    assert isinstance(in_flight_snapshot.get("task"), asyncio.Task)
+    # Post-flight: the entry has been cleaned up.
+    assert "chat_bot_a" not in _in_flight_tasks