feat: add ChatConversation and ChatMessage models

- Introduced ChatConversation and ChatMessage structs to represent conversation and message data in the application.
- Included relevant fields with JSON and BSON annotations for proper serialization and database integration.
- Enhanced model definitions with descriptions for better understanding of their purpose and usage.
- Ensured alignment with existing coding standards and practices for maintainability.

core/models/models/chat_conversation.go

@@ -0,0 +1,19 @@
+package models
+
+import (
+	"go.mongodb.org/mongo-driver/bson/primitive"
+)
+
+type ChatConversation struct {
+	any           `collection:"chat_conversations"`
+	BaseModel     `bson:",inline"`
+	Title         string             `json:"title" bson:"title" description:"Conversation title"`
+	Description   string             `json:"description,omitempty" bson:"description,omitempty" description:"Conversation description"`
+	UserId        primitive.ObjectID `json:"user_id" bson:"user_id" description:"User ID who owns this conversation"`
+	Model         string             `json:"model" bson:"model" description:"Default AI model for this conversation"`
+	Status        string             `json:"status" bson:"status" description:"Conversation status (active/archived/deleted)"`
+	LastMessageAt primitive.DateTime `json:"last_message_at,omitempty" bson:"last_message_at,omitempty" description:"Last message timestamp"`
+	Settings      map[string]any     `json:"settings,omitempty" bson:"settings,omitempty" description:"Conversation settings"`
+	Tags          []string           `json:"tags,omitempty" bson:"tags,omitempty" description:"Conversation tags"`
+	Messages      []*ChatMessage     `json:"messages,omitempty" bson:"-" description:"Messages in this conversation (populated field)"`
+}

core/models/models/chat_message.go

@@ -0,0 +1,18 @@
+package models
+
+import (
+	"go.mongodb.org/mongo-driver/bson/primitive"
+)
+
+type ChatMessage struct {
+	any            `collection:"chat_messages"`
+	BaseModel      `bson:",inline"`
+	ConversationId primitive.ObjectID `json:"conversation_id" bson:"conversation_id" description:"Conversation ID"`
+	Role           string             `json:"role" bson:"role" description:"Message role (user/assistant/system)"`
+	Content        string             `json:"content" bson:"content" description:"Message content"`
+	Tokens         int                `json:"tokens" bson:"tokens" description:"Number of tokens in the message"`
+	Model          string             `json:"model" bson:"model" description:"AI model used"`
+	Metadata       map[string]any     `json:"metadata,omitempty" bson:"metadata,omitempty" description:"Additional metadata"`
+	Status         string             `json:"status" bson:"status" description:"Message status (pending/completed/failed)"`
+	Error          string             `json:"error,omitempty" bson:"error,omitempty" description:"Error message if failed"`
+}