[chore] clear all 64 pre-existing mypy errors in tests/ (no type:ignore)

Convention: no type:ignore/noqa, and pre-existing violations still
violate. The make-quality gate runs 'mypy roboco/ tests/', but the
prior commits' gates only ran mypy on production files, masking 64
type errors across 15 test files (method-assign, unused-ignore,
no-untyped-def, attr-defined, union-attr, has-type, index, misc).

Fixed without any type:ignore:
- method-assign (svc.session.X = / svc.method = AsyncMock()): hold a
  local 'session: MagicMock'/'AsyncMock' and assert on it, or stub via
  object.__setattr__ / monkeypatch / a typed '_bind' helper returning
  Any, or alias 'cc: Any = c' (the pattern the file already used).
- unused 'type: ignore[assignment]' (real code was method-assign):
  removed; replaced with the no-suppression patterns above.
- 'Callable[...] has no attribute assert_*': keep a typed local ref to
  the AsyncMock and assert on the local, not the method-typed attr.
- no-untyped-def: annotate helper params (Any / pytest.MonkeyPatch).
- attr-defined / index / union-attr: type the helper as Any, narrow
  with an 'is not None' assert, or add the missing attr to a fake.
- has-type / return-value: fix the declared return type to the tuple
  the function actually returns.
- PLC0415 inline imports: hoisted to top-level.

test_pr_gate_notifies_pm._stub_gate_path converted fully to the
'cc: Any = c' alias (it already used it for one attr) so its five
'# type: ignore[method-assign]' suppressions are gone.

mypy tests/: 64 errors -> 0 (538 files). ruff check tests/: clean.
All 84 tests in the touched files pass.

tests/unit/services/test_messaging_channel_race.py

@@ -25,6 +25,15 @@ _SLUG = "backend-cell"
 _LOOKUPS_BEFORE_AND_AFTER_RACE = 2
 
 
+def _bind(svc: object, name: str, value: object) -> Any:
+    """Stub `name` on `svc` without tripping mypy's method-assign check.
+    Returns the value (typed ``Any``) so the caller can keep a reference for
+    assertions — ``object.__setattr__`` does not narrow the attribute type, so
+    assert on the returned local, not ``svc.<name>``."""
+    object.__setattr__(svc, name, value)
+    return value
+
+
 def _integrity_error() -> IntegrityError:
     return IntegrityError(
         "INSERT INTO channels ...",
@@ -55,14 +64,16 @@ async def test_race_lost_refetches_existing_channel() -> None:
     re-fetches the winner's channel and returns it (no crash)."""
     existing = MagicMock(name="existing-channel", slug=_SLUG)
     svc, session = _svc(flush_side_effect=_integrity_error())
-    svc.get_channel_by_slug = AsyncMock(side_effect=[None, existing])
+    get_channel_by_slug = _bind(
+        svc, "get_channel_by_slug", AsyncMock(side_effect=[None, existing])
+    )
 
     result = await svc.get_or_create_channel_by_slug(_SLUG)
 
     assert result is existing
     # Savepoint isolated the failed insert; re-fetch was the recovery.
     session.begin_nested.assert_called_once()
-    assert svc.get_channel_by_slug.await_count == _LOOKUPS_BEFORE_AND_AFTER_RACE
+    assert get_channel_by_slug.await_count == _LOOKUPS_BEFORE_AND_AFTER_RACE
 
 
 @pytest.mark.asyncio
@@ -70,7 +81,7 @@ async def test_race_lost_but_re_fetch_empty_reraises() -> None:
     """If the conflict did NOT produce a row on re-fetch (a real failure, not a
     race), the IntegrityError is re-raised — never masked as a silent None."""
     svc, _session = _svc(flush_side_effect=_integrity_error())
-    svc.get_channel_by_slug = AsyncMock(side_effect=[None, None])
+    _bind(svc, "get_channel_by_slug", AsyncMock(side_effect=[None, None]))
 
     with pytest.raises(IntegrityError):
         await svc.get_or_create_channel_by_slug(_SLUG)
@@ -82,8 +93,8 @@ async def test_normal_auto_create_unaffected_by_savepoint() -> None:
     newly-created channel is returned (regression guard — the savepoint must
     not break the happy path)."""
     svc, session = _svc()  # flush succeeds
-    svc.get_channel_by_slug = AsyncMock(
-        side_effect=[None]
+    get_channel_by_slug = _bind(
+        svc, "get_channel_by_slug", AsyncMock(side_effect=[None])
     )  # not present, then never re-called
 
     result = await svc.get_or_create_channel_by_slug(_SLUG)
@@ -93,4 +104,4 @@ async def test_normal_auto_create_unaffected_by_savepoint() -> None:
     session.begin_nested.assert_called_once()
     session.add.assert_called_once()
     assert session.flush.await_count == 1
-    assert svc.get_channel_by_slug.await_count == 1  # no recovery re-fetch
+    assert get_channel_by_slug.await_count == 1  # no recovery re-fetch