[Feature Request]Default model setting function for each bot

## Describe the solution you'd like

I would like to see a default model setting feature added for each bot.

## Why the solution needed

With the release of the practical Claude 4.5 Haiku, it is becoming increasingly important to switch between LLM models depending on the processing content.
I would like to fix the model depending on the bot.
Currently, even if the number of available models in the Bot Edit screen is reduced, the previously selected model will continue to be selected unless the user selects a model.

## Additional context

when checking the default setting in the model settings available on the bot editing screen, I want the LLM selection at the start of the bot to be the default value.

## Implementation feasibility

Are you willing to collaborate with us to discuss the solution, decide on the approach, and assist with the implementation?

- [ ] Yes, I am able to implement the feature and create a pull request.
- [x] No, I am unable to implement the feature, but I am open to discussing the solution.

Start at the Bot Edit screen and its model settings, then trace how the selected model is carried into bot startup. Define how a per-bot default is stored and applied when the bot begins, including the case where available models change; confirm the behavior with the project's existing tests or manual bot flow.

Written by the indexing model from the issue text.