Make character parameter required when AI character selection is enabled

- Add character to required parameters when setting is enabled
- Update documentation to clarify character is required
- Ensure handler logic provides default character if missing
- Maintain backward compatibility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

example.html

@@ -92,12 +92,12 @@ generateChatBubbleImage("I'm thinking...", "thought")</pre>
 
 The function parameters are:
 - text: The text to display in the chat bubble (required string)
-- character: The character to use for the bubble (optional string: "Example", "Bianca", etc.)
+- character: The character to use for the bubble (required string: "Example", "Bianca", etc.)
 - bubble_type: The type of bubble to use (optional string: "speech" or "thought")
 
-When you want to create a chat bubble, call this function with the text you want to display.
+When you want to create a chat bubble, call this function with the text and character.
 Example usage:
-generateChatBubbleImage("Hello world!")
+generateChatBubbleImage("Hello world!", "Example")
 generateChatBubbleImage("Hello from Bianca", "Bianca")
 generateChatBubbleImage("I'm thinking...", "Example", "thought")</pre>
                     <p><strong>ALTERNATIVE:</strong> If the AI can't call the function directly, you can instruct it to respond with Markdown formatted like this:</p>