core/lmproto: allow null assistant content and optional response fields

Providers return content=null with tool_calls; relax choice logprobs,
finish_reason, fingerprint, and usage for real API payloads.

src/plyngent/agent/loop.py

@@ -66,7 +66,7 @@ async def run_chat_loop(  # noqa: PLR0913
         messages.append(assistant)
         yield AssistantMessageEvent(message=assistant)
 
-        if assistant.content:
+        if isinstance(assistant.content, str) and assistant.content:
             yield TextDeltaEvent(content=assistant.content)
 
         tool_calls = assistant.tool_calls

src/plyngent/lmproto/openai_compatible/model.py

@@ -65,7 +65,9 @@ class AssistantCustomToolCall(Struct, tag_field="type", tag="custom"):
 type AnyAssistantToolCall = AssistantFunctionToolCall | AssistantCustomToolCall
 
 
-class AssistantChatMessage(ChatMessage, tag_field="role", tag="assistant"):
+class AssistantChatMessage(Struct, tag_field="role", tag="assistant"):
+    # APIs often return content=null when tool_calls are present.
+    content: str | None | Unset = UNSET
     name: str | Unset = UNSET
     audio: IDObject | Unset = UNSET
     refusal: str | Unset = UNSET
@@ -201,8 +203,8 @@ class ChatCompletionsParam(Struct):
 class ChatCompletionChoice(Struct):
     index: int
     message: AssistantChatMessage
-    logprobs: dict[str, Any]
-    finish_reason: FinishReason
+    logprobs: dict[str, Any] | None = None
+    finish_reason: FinishReason | None = None
 
 
 class ChatCompletionResponse(Struct):
@@ -211,8 +213,8 @@ class ChatCompletionResponse(Struct):
     created: int
     model: str
     choices: list[ChatCompletionChoice]
-    system_fingerprint: str
-    usage: dict[str, Any]
+    system_fingerprint: str | Unset = UNSET
+    usage: dict[str, Any] | Unset = UNSET
     moderation: dict[str, Any] | Unset = UNSET
     service_tier: ServiceTier | Unset = UNSET