feat: parse EXTRA_BODY as a JSON object per source

src/polygateway/config.py

@@ -11,6 +11,7 @@ fail-loud 校验语义与 pydantic-settings 一致。
 
 from __future__ import annotations
 
+import json
 import os
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
@@ -44,6 +45,7 @@ _SOURCE_FIELDS: dict[str, tuple[str, str]] = {
     "ENABLE_THINKING": ("enable_thinking", "bool"),
     "MISSING_DONE": ("missing_done", "str"),
     "TRUST_ENV": ("trust_env", "bool"),
+    "EXTRA_BODY": ("extra_body", "json"),
 }
 _RESERVED_SEGMENTS = frozenset({"GLOBAL", "RETRY", "BREAKER", "BACKPRESSURE"})
 _SELECTORS = frozenset({"round_robin", "least_inflight", "health_aware"})
@@ -74,6 +76,12 @@ def _cast(raw: str, kind: str, key: str) -> object:
             if lowered in ("0", "false", "no", "off"):
                 return False
             raise ValueError(f"非法布尔值: {raw!r}")
+        if kind == "json":
+            # JSONDecodeError 是 ValueError 子类,复用下方的统一包装
+            parsed = json.loads(raw)
+            if not isinstance(parsed, dict):
+                raise ValueError(f"必须是 JSON 对象(而非数组/标量): {raw!r}")
+            return parsed
         return raw
     except ValueError as exc:
         raise ValueError(f"配置 {key} 解析失败: {exc}") from exc