fix: parse_turn falls back gracefully + classify logs flapping classifiers

The turn endpoint was 500ing in multi-bot scenes whenever the
classifier provider hiccuped on parse_turn — particularly visible
after a guest was added and bots started exchanging turns. The
traceback was 'classify failed for schema ParsedTurn with no default'
because parse_turn was the only classify caller without a default.

Two changes:

- chat/services/turn_parse.py: parse_turn now passes a default that
  wraps the whole prose as one 'dialogue' segment. The narrative
  still fires on the prose; we lose finer-grained segment kinds
  (action vs dialogue vs ooc) on this turn, but the request returns
  cleanly. Updated the existing test that pinned the old
  RuntimeError contract.

- chat/llm/classify.py: when retries are exhausted, log a WARNING
  with the schema name, last error type, and a snippet of the last
  raw text the model returned. Surfaces flapping classifiers in the
  uvicorn log for diagnosis without taking down the request.

Suite: 471 passed in 11.7s.

tests/test_turn_parse.py

@@ -73,11 +73,25 @@ async def test_parse_turn_empty_prose_short_circuits_without_classifier_call():
 
 
 @pytest.mark.asyncio
-async def test_parse_turn_raises_when_classifier_fails_twice():
+async def test_parse_turn_falls_back_to_whole_prose_when_classifier_fails():
+    """A flapping classifier (3 invalid responses) no longer 500s the
+    request. ``parse_turn`` returns the original prose as a single
+    ``dialogue`` segment so the turn flow can keep moving — the
+    narrative will still fire on the prose, just without finer-grained
+    segment classification.
+
+    The old contract was ``RuntimeError`` (no default), but in
+    production that took down the whole turn endpoint with a 500 the
+    moment any classifier provider hiccuped — particularly painful in
+    multi-bot scenes where every user turn pays the parse_turn cost.
+    """
     mock = MockLLMClient(canned=["nope", "still nope", "nope3"])
-    with pytest.raises(RuntimeError):
-        await parse_turn(
-            mock,
-            model="m",
-            prose='*shrugs* "whatever"',
-        )
+    result = await parse_turn(
+        mock,
+        model="m",
+        prose='*shrugs* "whatever"',
+    )
+    assert len(result.segments) == 1
+    assert result.segments[0].kind == "dialogue"
+    assert result.segments[0].text == '*shrugs* "whatever"'
+    assert result.intent == "narrative"