fix missing finish_reason (#140)

Author: orangetin

src/together/types/completions.py

@@ -61,7 +61,7 @@ def verify_parameters(self) -> Self:
 class CompletionChoicesData(BaseModel):
     index: int
     logprobs: LogprobsPart | None = None
-    finish_reason: FinishReason
+    finish_reason: FinishReason | None = None
     text: str