feat(chat): removes feature flag requirement for title generation

[abeatrix]

This commit improves the chat title generation feature with the following changes:

• Generates a custom title only if the input text is longer than 20 characters.
• Uses the latest Gemini flash-lite model (if available) or the first speedy model as the default for title generation.
• Removes the feature flag check for auto-generating chat titles.

Test plan

1. Ask a question that has less than 20 characters to confirm chat title in chat history remains unchanged.
2. Ask a question that has more than 20 characters to see if chat title was generated automatically.
3. Instance without speed model is currently not supported. Not reason to use expensive or reasoning models for small task.