fix(headers): 兼容非 ASCII header 值，避免 httpx ASCII 编码报错

将 HeaderBuilder 中的 latin-1 透传方案替换为 ASCII 归一化策略：
- 对 x-codex-turn-metadata 做 JSON 重编码（ensure_ascii=true）保留语义
- 其他非 ASCII 头值仅转义非 ASCII 字符为 \uXXXX，保留 ASCII 字符原样
- 新增单测覆盖中文 header 与 codex turn metadata 场景

Close #180

Co-Authored-By: AAEE86 <ppk0227@hotmail.com>

tests/unit/test_headers.py

@@ -1,3 +1,5 @@
+import json
+
 from src.core.api_format import (
     CORE_REDACT_HEADERS,
     HeaderBuilder,
@@ -73,6 +75,23 @@ class TestHeaderBuilder:
         assert built["Authorization"] == "base"
         assert built["X-Test"] == "1"
 
+    def test_non_ascii_value_is_escaped_for_httpx(self) -> None:
+        builder = HeaderBuilder()
+        builder.add("X-Test", "桌面")
+        built = builder.build()
+        assert built["X-Test"] == r"\u684c\u9762"
+
+    def test_codex_turn_metadata_is_normalized_to_ascii_json(self) -> None:
+        builder = HeaderBuilder()
+        raw = '{"turn_id":"t1","workspaces":{"d:\\\\桌面\\\\123\\\\Aether":{"has_changes":true}}}'
+        builder.add("x-codex-turn-metadata", raw)
+        built = builder.build()
+
+        normalized = built["x-codex-turn-metadata"]
+        assert normalized.isascii()
+        parsed = json.loads(normalized)
+        assert "d:\\桌面\\123\\Aether" in parsed["workspaces"]
+
 
 class TestBuildUpstreamHeaders:
     def test_priority_and_drop_headers(self) -> None: