feat: carry the new observability fields through retry and cache

tests/unit/test_retry.py

@@ -194,6 +194,35 @@ class TestSuccessPath:
         assert (await limiter.source_stats("a")).tpm_used == 16
 
 
+class TestObservabilityPassthrough:
+    """issue #3: transport 采到的两个可观测字段必须原样上浮到 LLMResponse。"""
+
+    async def test_fields_reach_the_response(self):
+        result = TransportResult(
+            content="ok",
+            thinking="",
+            prompt_tokens=10,
+            completion_tokens=5,
+            usage_source="measured",
+            ttft_ms=12.0,
+            max_inter_token_ms=3.0,
+            raw={},
+            cached_prompt_tokens=64,
+            model_reported="MiniMax-Text-01-250321",
+        )
+        mw, *_ = _harness([_src("a")], [result])
+        resp = await mw(_REQ)
+        assert resp.cached_prompt_tokens == 64
+        assert resp.model_reported == "MiniMax-Text-01-250321"
+        # model 仍是配置别名: 真实版本是旁证,不顶替溯源主字段
+        assert resp.model == "m"
+
+    async def test_absent_fields_stay_none(self):
+        mw, *_ = _harness([_src("a")], [_ok()])
+        resp = await mw(_REQ)
+        assert resp.cached_prompt_tokens is None and resp.model_reported is None
+
+
 class TestRetryAndFailover:
     async def test_transient_switches_source_then_succeeds(self):
         mw, _, _, transport, sleep, _ = _harness(