feat: let every gateway error carry what the gateway said

Issue #10 Task 1: a rejected call's reason had nowhere to live. The
field goes on the base class because these errors all come from one HTTP
response - which class it is and what the peer said are orthogonal.

tests/unit/test_errors.py

@@ -34,6 +34,43 @@ class TestBaseShape:
         assert TransientError("429", retry_after_s=2.5).retry_after_s == 2.5
 
 
+class TestBodyText:
+    """issue #10: 非 2xx 的响应体摘要必须有承载处,否则拒绝理由事后不可查。"""
+
+    @pytest.mark.parametrize(
+        "cls", (PolyGatewayError, TransientError, SourceDeadError, RequestRejectedError)
+    )
+    def test_defaults_empty_and_accepts_summary(self, cls):
+        assert cls("boom").body_text == ""
+        assert cls("boom", body_text='{"error":{"code":"bad"}}').body_text == (
+            '{"error":{"code":"bad"}}'
+        )
+
+    def test_result_invalid_keeps_both_fields_apart(self):
+        """`body_text`(非 2xx 的拒绝理由)与 `raw_text`(2xx 的不可解析输出)不得混用。"""
+        exc = ResultInvalidError("bad json", raw_text="{oops", body_text="")
+        assert exc.raw_text == "{oops"
+        assert exc.body_text == ""
+
+    @pytest.mark.parametrize(
+        "exc",
+        (
+            AllSourcesExhausted(scope="llm", reason="stalled", retry_after_s=1.0),
+            CircuitOpenError(scope="llm", retry_after_s=1.0),
+            GovernanceBackendError("redis down", scope="llm"),
+        ),
+    )
+    def test_scope_level_errors_carry_no_body(self, exc):
+        """scope 级失败没有单一响应体可言,空串是如实表达而非噪音。"""
+        assert exc.body_text == ""
+
+    def test_body_text_does_not_leak_into_str(self):
+        """字段是旁路数据: 加了它不得改变任何既有异常的 str() 输出。"""
+        assert str(RequestRejectedError("qwen_1 请求被拒: 400", body_text="whatever")) == (
+            "qwen_1 请求被拒: 400"
+        )
+
+
 class TestResultInvalid:
     def test_carries_diagnosis(self):
         exc = ResultInvalidError(
@@ -134,9 +171,7 @@ class TestGovernanceBackendReason:
         assert "governance_backend_down" in SCOPE_REASONS
 
     def test_gateway_unavailable_accepts_the_new_reason(self):
-        exc = AllSourcesExhausted(
-            scope="LLM", reason="governance_backend_down", retry_after_s=0.0
-        )
+        exc = AllSourcesExhausted(scope="LLM", reason="governance_backend_down", retry_after_s=0.0)
         assert exc.reason == "governance_backend_down"
 
     def test_retry_after_default_is_non_zero(self):