[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/optimal_brain/test_playbooks_index.py

@@ -7,6 +7,9 @@ BaseIndexPlugin (shared, proven by the other 8 plugins) and runs live.
 
 from __future__ import annotations
 
+import asyncio
+from unittest.mock import AsyncMock, MagicMock
+
 from roboco.models.optimal import IndexType
 from roboco.services.optimal_brain.indexes.playbooks import PlaybooksIndexPlugin
 
@@ -44,8 +47,6 @@ def test_delete_playbook_removes_its_chunks_by_source() -> None:
     """F011: deleting a playbook removes its embedded chunks from the vector
     store by the playbook's source URI (idempotent — no-op if absent). A
     rejected/archived playbook must not stay retrievable in the PLAYBOOKS index."""
-    from unittest.mock import AsyncMock, MagicMock
-
     plugin = PlaybooksIndexPlugin.__new__(PlaybooksIndexPlugin)
     store = MagicMock()
     store.delete_by_source = AsyncMock(return_value=None)
@@ -54,8 +55,6 @@ def test_delete_playbook_removes_its_chunks_by_source() -> None:
     object.__setattr__(plugin, "_initialized", True)
     object.__setattr__(plugin, "_store", store)
 
-    import asyncio
-
     asyncio.run(plugin.delete_playbook("pb-42"))
 
     store.delete_by_source.assert_awaited_once_with("roboco://playbooks/pb-42")