When a custom/Ollama provider is used and reasoning_effort is set to 'none'
(or enabled: false), inject 'think': false into the request extra_body.
Ollama does not recognise the OpenRouter-style 'reasoning' extra_body field,
so thinking-capable models (Qwen3, etc.) generate <think> blocks regardless
of the reasoning_effort setting. This produces empty-response errors that
corrupt session state.
The fix adds a provider-specific block in _build_api_kwargs() that sets
think=false in extra_body whenever self.provider == 'custom' and reasoning
is explicitly disabled.
Closes#3191
3522a7aa13