test: prove a rejected call's reason reaches the telemetry table

Issue #10 Task 5, the acceptance claim. Before the fix this asserted
against 'qwen_1 请求被拒: 400' and failed on the first substring - which
is exactly what the downstream batch was left with. Uses the real body
from the issue, and checks the trailing code too, since a head-only cut
would drop the one field you quote when chasing the provider.

tests/integration/test_governance_stack.py

@@ -11,7 +11,12 @@ import sqlite3
 import httpx
 import pytest
 
-from polygateway import CircuitOpenError, GatewayClient, TransientError
+from polygateway import (
+    CircuitOpenError,
+    GatewayClient,
+    RequestRejectedError,
+    TransientError,
+)
 from polygateway.backends.memory.breaker import InMemoryGate
 from polygateway.backends.memory.cache import InMemoryCache
 from polygateway.backends.memory.limiter import InMemoryLimiter
@@ -173,6 +178,40 @@ class TestTelemetryAcrossPaths:
         assert len({cid for _, cid in rows}) == 2  # call_id 逐次独立
 
 
+class TestRejectionReasonIsQueryable:
+    """issue #10 的验收主张: 400 之后,网关说的话必须能在遥测表里查到。
+
+    下游一轮 1050 张影像的批处理里,1 张在读表格时收到 400 被判确定性失败,
+    事后"这张图到底哪里不合规"无从查起——响应体在 transport 翻译层就没了。
+    """
+
+    # issue #10 原文给出的真实响应体(一字不改)
+    _BODY = (
+        '{"error":{"message":"<400> ***.***.InvalidParameter: The image format is illegal '
+        'and cannot be opened","type":"invalid_request_error","param":"",'
+        '"code":"invalid_parameter_error"}}'
+    )
+
+    async def test_rejected_call_leaves_the_reason_in_telemetry(self, tmp_path):
+        recorder = SQLiteRecorder(tmp_path / "t.db")
+        client = _full_client(
+            lambda req: httpx.Response(400, content=self._BODY.encode()), telemetry=recorder
+        )
+
+        with pytest.raises(RequestRejectedError):
+            await client.chat([{"role": "user", "content": "hi"}])
+
+        recorder.close()
+        rows = sqlite3.connect(tmp_path / "t.db").execute("SELECT error FROM llm_calls").fetchall()
+        assert rows, "400 必须留下遥测行(遥测必录)"
+        errors = " ".join(r[0] or "" for r in rows)
+        # 修复前这里只有 "qwen_1 请求被拒: 400"——诊断信息一个字都不在
+        assert "InvalidParameter" in errors
+        assert "The image format is illegal" in errors
+        # 尾部的 code 才是向网关方追查的凭据,头部硬切正好会丢掉它
+        assert "invalid_parameter_error" in errors
+
+
 class TestStructuredThroughStack:
     async def test_feedback_reask_passes_through_governance(self):
         """重问经过内层治理: 第二次真实请求同样被限流/熔断记账。"""