llama_cpp server: fix to ChatCompletionRequestMessage

When I generate a client, it breaks because it fails to process the schema of ChatCompletionRequestMessage

These fix that:
- I think `Union[Literal["user"], Literal["channel"], ...]` is the same as Literal["user", "channel", ...]
- Turns out default value `Literal["user"]` isn't JSON serializable, so replace with "user"

llama_cpp/llama_types.py

@@ -58,7 +58,7 @@ class Completion(TypedDict):
 
 
 class ChatCompletionMessage(TypedDict):
-    role: Union[Literal["assistant"], Literal["user"], Literal["system"]]
+    role: Literal["assistant", "user", "system"]
     content: str
 
 class ChatCompletionChoice(TypedDict):

llama_cpp/server/app.py

@@ -215,8 +215,8 @@ def create_embedding(
 
 
 class ChatCompletionRequestMessage(BaseModel):
-    role: Union[Literal["system"], Literal["user"], Literal["assistant"]] = Field(
-        default=Literal["user"], description="The role of the message."
+    role: Literal["system", "user", "assistant"] = Field(
+        default="user", description="The role of the message."
     )
     content: str = Field(default="", description="The content of the message.")