fix(types): resolve pre-existing basedpyright errors in tests

Fix ~790 basedpyright errors across the test suite:
- Type stub subclasses of real production classes with super().__init__()
- Add missing generic type arguments and Dict[str, Any] annotations
- Add None guards before subscript/member access
- Adapt tests to production API changes (removed dead handlers,
  PersistentModelCache.get_default, _i18n_filter_added location)

tests/services/test_downloader.py

@@ -9,7 +9,7 @@ from py.services.downloader import Downloader
 
 
 class FakeStream:
-    def __init__(self, chunks: Sequence[Sequence] | Sequence[bytes]):
+    def __init__(self, chunks: Sequence[bytes | tuple[bytes, float]]):
         self._chunks = list(chunks)
 
     async def read(self, _chunk_size: int) -> bytes:
@@ -25,6 +25,7 @@ class FakeStream:
             payload = item[0]
             delay = item[1]
 
+        assert isinstance(payload, bytes)
         await asyncio.sleep(delay)
         return payload
 
@@ -84,11 +85,11 @@ def _build_downloader(responses, *, max_retries=0):
     downloader.max_retries = max_retries
     downloader.base_delay = 0
     fake_session = FakeSession(responses)
-    downloader._session = fake_session
+    downloader._session = fake_session  # pyright: ignore[reportAttributeAccessIssue]
     downloader._session_created_at = datetime.now()
     downloader._proxy_url = None
     async def _noop_create_session():
-        downloader._session = fake_session
+        downloader._session = fake_session  # pyright: ignore[reportAttributeAccessIssue]
         downloader._session_created_at = datetime.now()
         downloader._proxy_url = None
 
@@ -96,6 +97,13 @@ def _build_downloader(responses, *, max_retries=0):
     return downloader
 
 
+def _session(downloader: Downloader) -> FakeSession:
+    """Return the injected fake session, asserting the runtime invariant."""
+    session = downloader._session
+    assert isinstance(session, FakeSession)
+    return session
+
+
 @pytest.mark.asyncio
 async def test_download_file_preserves_incomplete_part_when_size_mismatch(tmp_path):
     target_path = tmp_path / "model" / "file.bin"
@@ -196,7 +204,7 @@ async def test_download_file_recovers_from_stall(tmp_path):
 
     assert success is True
     assert Path(result_path).read_bytes() == payload
-    assert downloader._session._get_calls == 2
+    assert _session(downloader)._get_calls == 2
     assert not Path(str(target_path) + ".part").exists()
 
 
@@ -224,8 +232,8 @@ async def test_download_file_resumes_after_incomplete_integrity_check(tmp_path):
 
     assert success is True
     assert Path(result_path).read_bytes() == b"abcdef"
-    assert downloader._session._get_calls == 2
-    assert downloader._session.requests[1]["headers"]["Range"] == "bytes=3-"
+    assert _session(downloader)._get_calls == 2
+    assert _session(downloader).requests[1]["headers"]["Range"] == "bytes=3-"
     assert not Path(str(target_path) + ".part").exists()
 
 
@@ -261,6 +269,6 @@ async def test_download_file_retries_redirected_url_when_range_not_honored(tmp_p
     assert success is True
     assert Path(result_path).read_bytes() == b"abcdef"
     assert first_response.released is True
-    assert downloader._session.requests[0]["headers"]["Range"] == "bytes=3-"
-    assert downloader._session.requests[1]["url"] == redirected_url
-    assert downloader._session.requests[1]["headers"]["Range"] == "bytes=3-"
+    assert _session(downloader).requests[0]["headers"]["Range"] == "bytes=3-"
+    assert _session(downloader).requests[1]["url"] == redirected_url
+    assert _session(downloader).requests[1]["headers"]["Range"] == "bytes=3-"