[F089] honest WorkSession agent_id nullability across the read path

The work_sessions.agent_id column is nullable=True with ondelete=SET
NULL — deleting an agent nulls the FK on every session it ever held. The
ORM annotation lied (Mapped[UUID] non-optional), the converter papered
over the lie (typing_cast to a non-optional UUID), and the response
model rejected None outright (WorkSessionResponse.agent_id: UUID). A
session whose agent had been deleted crashed the GET endpoint with a
pydantic ValidationError instead of serializing agent_id: null.

Make the read path honest end-to-end:
- WorkSessionTable.agent_id: Mapped[UUID | None] (matches the column).
- WorkSessionResponse.agent_id: UUID | None (serializes null, no crash).
- session_to_response passes agent_id via typing_cast('UUID | None', ...)
  to bridge SQLAlchemy's UUID[Any] to stdlib uuid.UUID while preserving
  None-ness (the cast stays for the same mypy-plugin reason every other
  field uses one; it no longer narrows away None).

WorkSessionCreate.agent_id stays UUID — at create time the claiming
agent is always known. The unused WorkSession pydantic read model is
left as-is (never materialized from a DB row). task.py:_needs_revision_dev
already None-guards ws.agent_id via to_python_uuid (returns None -> skip).

roboco/api/schemas/work_session.py

@@ -28,7 +28,9 @@ class WorkSessionResponse(BaseModel):
     id: UUID
     project_id: UUID
     task_id: UUID
-    agent_id: UUID
+    # Nullable in the DB (ondelete SET NULL when an agent is deleted); a session
+    # whose agent was deleted serializes agent_id as null, not a crash.
+    agent_id: UUID | None
 
     # Branch management
     branch_name: str
@@ -129,7 +131,7 @@ def session_to_response(session: "WorkSessionTable") -> WorkSessionResponse:
         id=typing_cast("UUID", session.id),
         project_id=typing_cast("UUID", session.project_id),
         task_id=typing_cast("UUID", session.task_id),
-        agent_id=typing_cast("UUID", session.agent_id),
+        agent_id=typing_cast("UUID | None", session.agent_id),
         branch_name=str(session.branch_name),
         base_branch=str(session.base_branch),
         target_branch=str(session.target_branch),

roboco/db/tables.py

@@ -816,7 +816,7 @@ class WorkSessionTable(Base):
         nullable=False,
         index=True,
     )
-    agent_id: Mapped[UUID] = mapped_column(
+    agent_id: Mapped[UUID | None] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("agents.id", ondelete="SET NULL"),
         nullable=True,

tests/unit/api/schemas/test_work_session_response.py

@@ -0,0 +1,65 @@
+"""session_to_response / session_to_summary honesty for a null agent_id.
+
+The ``work_sessions.agent_id`` column is ``nullable=True`` with
+``ondelete="SET NULL"`` — if an agent row is deleted, the FK nulls out on every
+session that agent ever held. The read path (``session_to_response`` ->
+``WorkSessionResponse``) used to assume ``agent_id`` was always present: the
+ORM ``Mapped[UUID]`` lied, the converter ``typing_cast("UUID", ...)``
+papered over the lie, and ``WorkSessionResponse.agent_id: UUID`` rejected
+``None`` outright. A session whose agent had been deleted therefore crashed
+the GET endpoint with a pydantic ``ValidationError`` instead of serializing
+``agent_id: null``. These tests pin the honest end-to-end read path.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+from uuid import UUID, uuid4
+
+from roboco.api.schemas.work_session import (
+    WorkSessionResponse,
+    session_to_response,
+    session_to_summary,
+)
+from roboco.db.tables import WorkSessionTable
+from roboco.models.work_session import WorkSessionStatus
+
+
+def _make_session(agent_id: UUID | None) -> WorkSessionTable:
+    """Build a detached WorkSessionTable row with an explicit agent_id."""
+    now = datetime.now(UTC)
+    return WorkSessionTable(
+        id=uuid4(),
+        project_id=uuid4(),
+        task_id=uuid4(),
+        agent_id=agent_id,
+        branch_name="feature/x",
+        base_branch="main",
+        target_branch="main",
+        started_at=now,
+        status=WorkSessionStatus.ACTIVE,
+        commits=[],
+        files_modified=[],
+        created_at=now,
+    )
+
+
+def test_session_to_response_serializes_null_agent_id() -> None:
+    """A SET-NULL'd session (agent deleted) must serialize agent_id as None."""
+    session = _make_session(agent_id=None)
+    result = session_to_response(session)
+    assert isinstance(result, WorkSessionResponse)
+    assert result.agent_id is None
+
+
+def test_session_to_response_preserves_present_agent_id() -> None:
+    """A normal session still carries its agent_id through unchanged."""
+    agent_id = uuid4()
+    result = session_to_response(_make_session(agent_id=agent_id))
+    assert result.agent_id == agent_id
+
+
+def test_session_to_summary_does_not_read_agent_id() -> None:
+    """The summary view omits agent_id entirely, so a null agent must not raise."""
+    result = session_to_summary(_make_session(agent_id=None))
+    assert result.status == WorkSessionStatus.ACTIVE