fix: harden the observability fields against the verifier findings

src/polygateway/transports/openai_compat.py

@@ -45,9 +45,12 @@ def _sse_delta(chunk: dict[str, Any], usage_sink: dict[str, Any]) -> tuple[bool,
     """从 chunk 提取增量: (True, content) 或 (False, reasoning);usage 帧旁路进 sink。"""
     if chunk.get("usage"):
         usage_sink["usage"] = chunk["usage"]
-    if "model" not in usage_sink and chunk.get("model") is not None:
-        # 首次写入即固定: 末帧的异常值不得覆盖首帧报的真实版本(issue #3)
-        usage_sink["model"] = chunk["model"]
+    if "model" not in usage_sink:
+        # 首个**有效**值即固定: 末帧的异常值不得覆盖它;但首帧报空串也不能锁死
+        # sink——否则后续真实版本会丢(issue #3)
+        reported = _coerce_model_reported(chunk.get("model"))
+        if reported is not None:
+            usage_sink["model"] = reported
     choices = chunk.get("choices") or []
     if not choices:
         return None