feat: accept sampling overlay on chat() and per-source extra_body

Priority is structured injection > per-call overlay > per-source config,
and extra_body now takes part in the cache fingerprint.

src/polygateway/transports/openai_compat.py

@@ -294,6 +294,9 @@ class OpenAICompatTransport:
             payload.update(profile.thinking_on)
         elif source.enable_thinking is False:
             payload.update(profile.thinking_off)
+        # 顺序即优先级(issue #4 设计决策 A): 配置级 extra_body 在前,调用级
+        # overlay(含结构化注入)在后覆盖之。两行不可调换
+        payload.update(source.extra_body)
         payload.update(overlay)
         return payload