refactor(compression): 请求压缩策略改为跟随客户端行为，响应支持gzip压缩

- 移除全局 ENABLE_REQUEST_COMPRESSION 配置，改为根据客户端 Content-Encoding
  决定是否对上游请求体进行 gzip 压缩
- 非流式响应根据客户端 Accept-Encoding 返回 gzip 压缩的 JSON
- ApiRequestContext 记录客户端编码偏好并透传至 handler 链路
- 新增 http_compression 模块统一处理压缩相关判断逻辑
- 上游请求头丢弃列表新增 content-encoding 防止客户端值泄露
- ensure_json_body 支持解压 gzip 编码的请求体

tests/unit/test_headers.py

@@ -101,6 +101,7 @@ class TestBuildUpstreamHeaders:
                 "X-Api-Key": "client",
                 "User-Agent": "ua",
                 "Content-Type": "text/plain",
+                "Content-Encoding": "gzip",
             },
             "openai:chat",
             "provider",
@@ -112,6 +113,7 @@ class TestBuildUpstreamHeaders:
         assert result["Authorization"] == "Bearer provider"
         assert result["User-Agent"] == "extra"
         assert result["Content-Type"] == "text/plain"
+        assert "Content-Encoding" not in result
         assert result["X-Endpoint"] == "1"
         assert result["X-Extra"] == "1"