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/app/plex/stream.py

@@ -745,8 +745,7 @@ def start_session(
         # check-and-replace happens under ONE _lock, so a newer same-key session (a concurrent
         # seek-restart) installed between the fallback's decision and here is never clobbered. When the
         # registered session is no longer `replace_only`, we abort and return None (the caller yields to
-        # whoever is registered now). The INITIAL spawn passes replace_only=None and always supersedes —
-        # that is the seek-restart's intended last-writer-wins.
+        # whoever is registered now). The INITIAL spawn never gets None — see its call site.
         # Software-fallback encode tuning — read only on the SW transcode path (the VAAPI path and
         # remux ignore preset/crf/max_height, and remux must not touch the DB for them).
         preset, crf, max_height = "veryfast", 20, 0
@@ -823,6 +822,7 @@ def start_session(
         return s
 
     s = _spawn(*_select_encode(db, item, transcode, src))
+    assert s is not None  # replace_only=None always supersedes: a seek-restart's last-writer-wins
     # Wait for the first video segment outside the lock (this blocks on ffmpeg, and we must not hold up
     # other sessions). Pass the proc so a HW session that dies at VAAPI init fails fast (not after 25s).
     ready = wait_for(_seg(0), timeout=25.0, proc=s.proc)