core/deepseek: inject thinking parameter to satisfy API reasoning mode requirement

DeepSeek's reasoning model requires the `thinking` parameter to be
explicitly set in every request (default is enabled). The agent loop
constructs the base ChatCompletionsParam which lacks this field, so the
DeepseekOpenAIClient now injects `thinking: {type: "enabled"}` into the
JSON request body after msgspec encoding.

Fixes the HTTP 400 error:
"The `reasoning_content` in the thinking mode must be passed back to the API."
which occurred when a cancelled turn was retried and the request history
contained assistant messages with reasoning_content but no thinking param.

src/plyngent/lmproto/deepseek/openai_compat/client.py

@@ -1,3 +1,6 @@
+from __future__ import annotations
+
+import json
 from typing import TYPE_CHECKING, Literal, overload
 
 import msgspec
@@ -13,6 +16,19 @@ if TYPE_CHECKING:
     from .model import ChatCompletionsParam
 
 
+def _inject_thinking(data: bytes) -> bytes:
+    """Inject ``thinking: {type: "enabled"}`` into the encoded request body.
+
+    DeepSeek's reasoning model requires the ``thinking`` parameter to be
+    explicitly set (default is ``enabled``). The agent loop constructs the
+    base ``ChatCompletionsParam`` which lacks this field, so we add it
+    after msgspec encoding.
+    """
+    body = json.loads(data)
+    body["thinking"] = {"type": "enabled"}
+    return json.dumps(body, separators=(",", ":")).encode("utf-8")
+
+
 class DeepseekOpenAIClient(BaseOpenAIClient):
     def __init__(self, config: OpenAIConfig) -> None:
         super().__init__(config)
@@ -31,7 +47,7 @@ class DeepseekOpenAIClient(BaseOpenAIClient):
         self, param: ChatCompletionsParam, *, stream: bool = False
     ) -> ChatCompletionResponse | AsyncIterator[ChatCompletionChunk]:
         param = coerce_chat_completions_param_any(msgspec.structs.replace(param, stream=stream))
-        data = self.encoder.encode(param)
+        data = _inject_thinking(self.encoder.encode(param))
         if stream:
             resp = await self.session.post(
                 "/chat/completions",