fix(request-body): 使用 deepcopy 防止请求体在处理流程中被意外修改

handler 基类和格式转换 registry 中，原始请求体通过浅拷贝或直接引用传递，
导致下游处理（模型映射、格式转换、重试整流）可能修改原始数据，
影响后续重试或并发请求的正确性。统一改用 copy.deepcopy 隔离副本。

tests/core/api_format/conversion/test_registry_non_mutation.py

@@ -0,0 +1,132 @@
+from __future__ import annotations
+
+import copy
+from typing import Any
+
+from src.core.api_format.conversion.internal import (
+    FormatCapabilities,
+    InternalMessage,
+    InternalRequest,
+    InternalResponse,
+    Role,
+    TextBlock,
+)
+from src.core.api_format.conversion.normalizer import FormatNormalizer
+from src.core.api_format.conversion.registry import FormatConversionRegistry
+
+
+class _BaseTestNormalizer(FormatNormalizer):
+    capabilities = FormatCapabilities()
+
+    def response_to_internal(self, response: dict[str, Any]) -> InternalResponse:
+        return InternalResponse(id=str(response.get("id") or ""), model="", content=[])
+
+    def response_from_internal(
+        self,
+        internal: InternalResponse,
+        *,
+        requested_model: str | None = None,
+    ) -> dict[str, Any]:
+        return {
+            "id": internal.id,
+            "model": requested_model or internal.model,
+        }
+
+
+class _SameFormatNormalizer(_BaseTestNormalizer):
+    FORMAT_ID = "TEST:SAME"
+
+    def request_to_internal(self, request: dict[str, Any]) -> InternalRequest:
+        return InternalRequest(model=str(request.get("model") or ""), messages=[])
+
+    def request_from_internal(
+        self,
+        internal: InternalRequest,
+        *,
+        target_variant: str | None = None,
+    ) -> dict[str, Any]:
+        return {"model": internal.model, "variant": target_variant}
+
+
+class _MutatingSourceNormalizer(_BaseTestNormalizer):
+    FORMAT_ID = "TEST:MUTSRC"
+
+    def request_to_internal(self, request: dict[str, Any]) -> InternalRequest:
+        request.pop("ephemeral", None)
+        request["messages"][0]["content"][0]["text"] = "mutated"
+        return InternalRequest(
+            model=str(request.get("model") or ""),
+            messages=[
+                InternalMessage(
+                    role=Role.USER,
+                    content=[TextBlock(text=str(request["messages"][0]["content"][0]["text"]))],
+                )
+            ],
+        )
+
+    def request_from_internal(
+        self,
+        internal: InternalRequest,
+        *,
+        target_variant: str | None = None,
+    ) -> dict[str, Any]:
+        return {"model": internal.model, "variant": target_variant}
+
+
+class _TargetNormalizer(_BaseTestNormalizer):
+    FORMAT_ID = "TEST:MUTTGT"
+
+    def request_to_internal(self, request: dict[str, Any]) -> InternalRequest:
+        return InternalRequest(model=str(request.get("model") or ""), messages=[])
+
+    def request_from_internal(
+        self,
+        internal: InternalRequest,
+        *,
+        target_variant: str | None = None,
+    ) -> dict[str, Any]:
+        text = ""
+        if internal.messages and internal.messages[0].content:
+            first = internal.messages[0].content[0]
+            if isinstance(first, TextBlock):
+                text = first.text
+        return {"model": internal.model, "text": text, "variant": target_variant}
+
+
+def test_convert_request_same_format_returns_detached_copy() -> None:
+    registry = FormatConversionRegistry()
+    registry.register(_SameFormatNormalizer())
+
+    original = {
+        "model": "gpt-test",
+        "messages": [{"role": "user", "content": [{"type": "text", "text": "hello"}]}],
+    }
+
+    out = registry.convert_request(original, "test:same", "test:same")
+
+    assert out == original
+    assert out is not original
+    assert out["messages"] is not original["messages"]
+
+    out["messages"][0]["content"][0]["text"] = "changed"
+
+    assert original["messages"][0]["content"][0]["text"] == "hello"
+
+
+def test_convert_request_cross_format_does_not_mutate_original_input() -> None:
+    registry = FormatConversionRegistry()
+    registry.register(_MutatingSourceNormalizer())
+    registry.register(_TargetNormalizer())
+
+    original = {
+        "model": "gpt-test",
+        "ephemeral": "keep-me",
+        "messages": [{"role": "user", "content": [{"type": "text", "text": "hello"}]}],
+    }
+    snapshot = copy.deepcopy(original)
+
+    out = registry.convert_request(original, "test:mutsrc", "test:muttgt")
+
+    assert out["model"] == "gpt-test"
+    assert out["text"] == "mutated"
+    assert original == snapshot