[fix] resolve 16 mypy errors across 9 test files (make quality gate)

type-clean the test files so make quality (mypy roboco/ tests/) is green:
- Any-typed locals for the two TypeError-asserting scoping tests (bypass
  the required-arg check without getattr/ruff B009)
- Any-typed view for the shutdown-drain _drain_bg_tasks override (bypass
  mypy method-assign without setattr/ruff B010)
- cast("uuid.UUID", ...) / cast("UUID", ...) for SQLAlchemy UUID[Any]
  returns (TC006-quoted), config=None for AgentInstance stubs, None-narrowed
  await_args, Iterator return on a yielding fixture, UUID annotation on the
  _task helper. No type:ignore / noqa.

tests/unit/services/test_git_pr_target_scoping.py

@@ -90,8 +90,11 @@ async def test_pr_target_requires_project_id() -> None:
     recorder: list[object] = []
     svc = _service(recorder)
 
+    # Bind to an Any-typed local so mypy doesn't flag the missing project_id;
+    # the call still reaches the runtime, where it raises TypeError as asserted.
+    target: Any = svc.pr_target
     with pytest.raises(TypeError):
-        await svc.pr_target(_PR_NUMBER)  # missing required project_id
+        await target(_PR_NUMBER)
 
     # The unscoped lookup was never issued — no SQL reached the session.
     assert recorder == []