fix: harden the observability fields against the verifier findings

tests/unit/test_openai_compat.py

@@ -360,6 +360,19 @@ class TestObservabilityFields:
         result = await _complete(_transport_for(handler), _source())
         assert result.model_reported == "MiniMax-Text-01-250321"
 
+    async def test_empty_first_model_does_not_block_a_later_real_one(self):
+        """首帧报空串不得锁死 sink: 守卫按"有效值"判断,否则真实版本会丢。"""
+
+        def handler(request):
+            return _sse_stream(
+                _chunk(content="a", model=""),
+                _chunk(content="b", model="MiniMax-Text-01-250321"),
+                _chunk(usage=_USAGE),
+            )
+
+        result = await _complete(_transport_for(handler), _source())
+        assert result.model_reported == "MiniMax-Text-01-250321"
+
     @pytest.mark.parametrize("bad", [None, "", "   ", 123, {}])
     async def test_missing_or_malformed_model_is_none(self, bad):
         def handler(request):