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.

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