feat: add the single summarizer for HTTP error bodies

Issue #10 Task 2: head-and-tail rather than a head-only cut, because the
code and request_id that let you chase the provider sit at the very end
of a JSON error body. Cap 2048 follows k8s client-go for the same job.

tests/unit/test_http_error_body.py

@@ -0,0 +1,89 @@
+"""HTTP 错误响应体摘要口径(issue #10 设计 §3.2/§3.4)。
+
+摘要是 message 与 `body_text` 共用的**同一份串**,故它的边界行为直接决定
+遥测里看到的与下游 catch 到的是否一致——本组用例把规则钉成算术。
+"""
+
+import httpx
+import pytest
+
+from polygateway.transports._http_errors import (
+    _ERROR_BODY_CAP,
+    _HEAD_CHARS,
+    _TAIL_CHARS,
+    compose_message,
+    response_body,
+    summarize_body,
+)
+
+# issue #10 原文给出的真实响应体(一字不改),关键在于 code 收尾
+_REAL_SAMPLE = (
+    '{"error":{"message":"<400> ***.***.InvalidParameter: The image format is illegal '
+    'and cannot be opened","type":"invalid_request_error","param":"",'
+    '"code":"invalid_parameter_error"}}'
+)
+
+
+class TestSummarizeBody:
+    def test_short_body_passes_through(self):
+        assert summarize_body(_REAL_SAMPLE) == _REAL_SAMPLE
+
+    def test_whitespace_collapsed(self):
+        """错误体常是缩进 JSON: 不折叠会把一行日志炸成多行、遥测列不可读。"""
+        assert (
+            summarize_body('{\n  "error": {\n    "code": "x"\n  }\n}')
+            == '{ "error": { "code": "x" } }'
+        )
+
+    @pytest.mark.parametrize("raw", ("", "   ", "\n\t  \n"))
+    def test_blank_yields_empty(self, raw):
+        assert summarize_body(raw) == ""
+
+    def test_exactly_at_cap_is_untouched(self):
+        body = "x" * _ERROR_BODY_CAP
+        assert summarize_body(body) == body
+
+    def test_one_over_cap_is_summarized(self):
+        summary = summarize_body("x" * (_ERROR_BODY_CAP + 1))
+        assert summary != "x" * (_ERROR_BODY_CAP + 1)
+        assert "略" in summary
+
+    def test_head_and_tail_both_survive(self):
+        """头部硬切会丢掉尾部,而 JSON 错误体的 code/request_id 正在尾部。"""
+        body = "H" * 5000 + "T" * 5000
+        summary = summarize_body(body)
+        assert summary[:_HEAD_CHARS] == body[:_HEAD_CHARS]
+        assert summary[-_TAIL_CHARS:] == body[-_TAIL_CHARS:]
+        assert f"…(略 {10000 - _HEAD_CHARS - _TAIL_CHARS} 字)…" in summary
+
+    def test_real_sample_tail_visible_in_oversized_body(self):
+        """设计 §7 用例 3c: 超长体里,追查网关方所需的 code 仍须可见。"""
+        summary = summarize_body("PADDING" * 1000 + _REAL_SAMPLE)
+        assert '"code":"invalid_parameter_error"}}' in summary
+
+    def test_idempotent(self):
+        """再摘要一次不得嵌套标记,否则重复经手的串会层层套娃。"""
+        once = summarize_body("y" * 9999)
+        assert summarize_body(once) == once
+
+
+class TestComposeMessage:
+    def test_empty_summary_leaves_message_intact(self):
+        assert compose_message("qwen_1 请求被拒: 400", "") == "qwen_1 请求被拒: 400"
+
+    def test_non_empty_summary_is_appended(self):
+        assert compose_message("qwen_1 请求被拒: 400", "{}") == "qwen_1 请求被拒: 400 | {}"
+
+
+class TestResponseBody:
+    def test_reads_buffered_text(self):
+        assert response_body(httpx.Response(400, content=b'{"e":1}')) == '{"e":1}'
+
+    def test_unread_stream_degrades_to_empty(self):
+        """取不到诊断信息绝不能升级为崩溃: 未读缓冲返回空串,且不触发网络读。"""
+
+        class _Unread(httpx.SyncByteStream):
+            def __iter__(self):
+                yield b"body"
+
+        assert response_body(httpx.Response(400, stream=_Unread())) == ""