chore: improve message handling by trimming whitespace in content checks

### CHANGES

- Remove default space in `BuildSession` message content
- Trim whitespace in `anthropic` message content check
- Trim whitespace in `gemini` message content check

internal/core/chatter.go

@@ -175,7 +175,7 @@ func (o *Chatter) BuildSession(request *domain.ChatRequest, raw bool) (session *
 	if request.Message == nil {
 		request.Message = &chat.ChatCompletionMessage{
 			Role:    chat.ChatMessageRoleUser,
-			Content: " ",
+			Content: "",
 		}
 	}
 

internal/plugins/ai/anthropic/anthropic.go

@@ -356,7 +356,7 @@ func (an *Client) toMessages(msgs []*chat.ChatCompletionMessage) (ret []anthropi
 	lastRoleWasUser := false
 
 	for _, msg := range msgs {
-		if msg.Content == "" {
+		if strings.TrimSpace(msg.Content) == "" {
 			continue // Skip empty messages
 		}
 

internal/plugins/ai/gemini/gemini.go

@@ -456,7 +456,7 @@ func (o *Client) convertMessages(msgs []*chat.ChatCompletionMessage) []*genai.Co
 			content.Role = "user"
 		}
 
-		if msg.Content != "" {
+		if strings.TrimSpace(msg.Content) != "" {
 			content.Parts = append(content.Parts, &genai.Part{Text: msg.Content})
 		}