test: no test throws a response away; the stream session is narrowed, not baselined

The 7 baselined Optional accesses in plex/stream.py were one fact the type did not carry: the
initial spawn (replace_only=None) always supersedes, so only the fallback respawn can get None.
An assert at the call site says so; the pyright baseline drops 195 -> 188.

test_abuse_limits: the public-URL and YouTube enqueue tests stopped at the profile lookup
(400 no_profile) and passed on that unrelated refusal. They now send an inline spec and assert
the job is queued. The same shape — a request whose response is discarded — sat at 53 more
places across the suite (all returned what they should, measured with a TestClient probe);
each now asserts its status, and test_response_checked keeps it that way (seen red).

backend/tests/test_plex_image_sizes.py

@@ -67,7 +67,7 @@ def test_each_variant_is_cut_at_its_own_size(db, make_user, monkeypatch, tmp_pat
     ]
     # Each cut is cached under its own name: asking again reaches Plex no more.
     for variant in ("thumb", "thumb_sm", "art", "art_sm"):
-        client.get(f"/api/plex/image/pm1?variant={variant}")
+        assert client.get(f"/api/plex/image/pm1?variant={variant}").status_code == 200
     assert len(_FakePlex.calls) == 4