[chore] mypy tests/: clear all 15 pre-existing type errors so make quality can go green

The branch tip had 15 mypy tests/ errors in files this bundle did not author,
which blocked CI's make quality mypy step (mypy roboco/ tests/) regardless of
the bundle's own commits. Pre-existing is still existing — fix every one:

- test_schemas_v1_flow.py (8): the StrList coercion tests intentionally pass
  SDK-nested list-of-strings input ([[['...']]], {'item':{'$text':'...'}}, int,
  dict). Annotate those literals as list[Any] locals so mypy accepts the
  coerce-able shape; the StrList BeforeValidator still flattens to list[str] at
  runtime. No type:ignore.
- test_pr_gate_records_verdict.py (3): notes_structured is dict|None; narrow
  with 'assert t.notes_structured is not None' before indexing (the existing
  pattern at line 90).
- test_pr_review_hand_format_guard.py (1 site, 2 errors): the _verb_runner()
  spy assertion — use the cc: Any = c alias idiom so assert_not_awaited
  resolves; drops the now-unused type:ignore[union-attr].
- test_pr_gate_notifies_pm.py (1): drop the unused type:ignore[method-assign]
  on the a2a.send reassignment.
- test_content_models.py (1): narrow coerced with isinstance(coerced,
  PrReviewContent) before reading .issues (the base _Content lacks the field).

Gates: rm -rf .mypy_cache && mypy roboco/ tests/ = Success (855 files);
ruff check + format clean; 5 affected suites = 40 passed.

tests/unit/gateway/test_pr_gate_notifies_pm.py

@@ -233,7 +233,7 @@ async def test_pr_fail_a2a_failure_is_swallowed() -> None:
 
     c = _make_choreographer()
     _stub_gate_path(c, reviewer_id=reviewer_id, t_before=t_before, t_after=t_after)
-    c.a2a.send = AsyncMock(side_effect=RuntimeError("db hiccup"))  # type: ignore[method-assign]
+    c.a2a.send = AsyncMock(side_effect=RuntimeError("db hiccup"))
 
     env = await c.pr_fail(reviewer_id, task_id, ["a concrete actionable issue"])
     # Verdict still landed — the owning PM is in needs_revision.