[FEATURE] Auto Mode for selecting a model in AI Assistant

## Feature Summary

Currently, users of AKS Desktop must manually select a specific AI model (e.g., gpt-5.4, gpt-4o, claude-opus-4.6, o3, o4-mini) from the dropdown menu for cluster troubleshooting and natural language queries.

## Motivation

1. Eliminate Guesswork
2. Optimize Performance & Latency
3. Cost & Token Optimization
4. Consistency across Ecosystem

## Detailed Description

Having a similar setup as GitHub Copilot, user can use Auto mode and AI assistant will select the suitable model for you

We request adding an Auto model selection option at the top of the dropdown list. This feature will automatically route user prompts to the optimal AI model based on query complexity, model availability, latency, and cost efficiency.

## Use Cases

As a Kubernetes administrator, I want the chat assistant to automatically select the optimal AI model based on my query complexity so that I can get instant answers to simple cluster checks without waiting for heavy reasoning models to process.

## Checklist

- [x] I have searched for existing feature requests
- [x] I have provided clear use cases
- [x] I have considered alternatives
- [x] This feature aligns with the project's goals

The issue does not name files or tests. Start by locating the AI Assistant model dropdown and the existing model-selection path; define the routing criteria, fallback behavior, and UI state before implementing and validating automatic selection across the listed use cases.

Written by the indexing model from the issue text.