test(v1): align tests with valid-chunk envelope and leak-strip signature

- test_standard_clients_can_ignore_docsgpt: docsgpt-only events now ride
  on a valid empty chat.completion.chunk (choices[0].delta == {}) so
  strict OpenAI clients accept the frame; assert the envelope plus the
  docsgpt extension instead of the absence of choices.
- split continuation persistence into stateful (conversation_id present
  -> save) and stateless (no conversation_id -> no save) cases.
- _fake_translate_stream_event mock accepts strip_reasoning_leak kwarg.

tests/api/v1/test_routes_extended.py

@@ -541,7 +541,9 @@ class TestChatCompletionsHappyPath:
 
         translated_chunks: list = []
 
-        def _fake_translate_stream_event(event_data, completion_id, model_name):
+        def _fake_translate_stream_event(
+            event_data, completion_id, model_name, strip_reasoning_leak=False
+        ):
             translated_chunks.append(event_data)
             return ['data: x\n\n']