[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_git_merge_method_fallback.py

@@ -181,13 +181,13 @@ async def test_merge_already_merged_pr_is_idempotent_success(
         svc, "_get_project_token_or_raise", AsyncMock(return_value="tok")
     )
     monkeypatch.setattr(svc, "_parse_github_remote", lambda _ws: ("owner", "repo"))
-    monkeypatch.setattr(
-        svc, "_delete_pr_branch_best_effort", AsyncMock(return_value=None)
-    )
+    delete_branch = AsyncMock(return_value=None)
+    monkeypatch.setattr(svc, "_delete_pr_branch_best_effort", delete_branch)
     monkeypatch.setattr(
         svc, "_project_default_branch", AsyncMock(return_value="master")
     )
-    monkeypatch.setattr(svc, "_sync_target_branch", AsyncMock(return_value="abc123"))
+    sync_target = AsyncMock(return_value="abc123")
+    monkeypatch.setattr(svc, "_sync_target_branch", sync_target)
     # No fallback retry would help (already merged => 405 either way); make the
     # fallback lookup return None so the code falls straight through to the
     # already-merged disambiguation.
@@ -208,8 +208,8 @@ async def test_merge_already_merged_pr_is_idempotent_success(
     assert commit == "abc123"
     # The post-merge steps still run (branch cleanup, target sync) so the
     # caller's state stays consistent with "the PR is merged".
-    svc._delete_pr_branch_best_effort.assert_awaited_once()
-    svc._sync_target_branch.assert_awaited_once()
+    delete_branch.assert_awaited_once()
+    sync_target.assert_awaited_once()
 
 
 @pytest.mark.asyncio
@@ -224,9 +224,8 @@ async def test_merge_raises_when_not_merged_and_refused(
         svc, "_get_project_token_or_raise", AsyncMock(return_value="tok")
     )
     monkeypatch.setattr(svc, "_parse_github_remote", lambda _ws: ("owner", "repo"))
-    monkeypatch.setattr(
-        svc, "_delete_pr_branch_best_effort", AsyncMock(return_value=None)
-    )
+    delete_branch = AsyncMock(return_value=None)
+    monkeypatch.setattr(svc, "_delete_pr_branch_best_effort", delete_branch)
     monkeypatch.setattr(
         svc, "_project_default_branch", AsyncMock(return_value="master")
     )
@@ -244,4 +243,4 @@ async def test_merge_raises_when_not_merged_and_refused(
         await svc.merge_pull_request(Path("/tmp/ws"), 42, "squash", "proj")
 
     # No post-merge cleanup ran — the merge did not land.
-    svc._delete_pr_branch_best_effort.assert_not_awaited()
+    delete_branch.assert_not_awaited()