[Qwen/qwen3.5-9b] Reasoning setting 'medium' not supported - fallback to 'on'

Issue Description

A warning message appears in the developer console when using the qwen/qwen3.5-9b model via LLM Studio:

2026-07-11 10:25:32 [WARN] Reasoning setting 'medium' is not supported by model 'qwen/qwen3.5-9b'. Supported settings: 'on', 'off'. Falling back to reasoning setting 'on'.

Details

• Model: qwen/qwen3.5-9b
• Error: The UI allows selecting 'medium' as a reasoning setting, but this model only supports 'on' and 'off'
• Impact: When a user selects 'medium', the system falls back to 'on' automatically

Expected Behavior

The reasoning dropdown should either:

1. Only show 'on' and 'off' for this specific model
2. Show a clearer error message instead of silently falling back
3. Validate the selected setting before applying it

This creates confusion when users see their selection not being honored, as they might think something else is wrong with their setup.