fix: add json struct tags to Message for API serialization

Message is used as a type alias by go-ml. Without json tags,
HTTP backends that serialize messages to JSON (e.g. OpenAI API)
would produce incorrect field names.

Co-Authored-By: Virgil <virgil@lethean.io>

inference.go

@@ -79,8 +79,8 @@ type Token struct {
 
 // Message represents a chat message for multi-turn conversation.
 type Message struct {
-	Role    string // "system", "user", "assistant"
-	Content string
+	Role    string `json:"role"`    // "system", "user", "assistant"
+	Content string `json:"content"`
 }
 
 // ClassifyResult holds the output for a single prompt in a batch classification.