* feat(providers): allow turning thinking off
Expose Off only for models that support disabled thinking while preserving Low as the default.
* fix(providers): reset unsupported thinking on model change
Keep the selected thinking option only when the new model advertises it; otherwise use that model's default and persist the runtime change.
* fix(providers): isolate custom model capabilities
Use strict manifest identity when reconciling thinking options so provider-prefixed custom models cannot inherit capabilities they do not advertise.
* fix(providers): clear thinking with default model
* fix(providers): validate disabled thinking support
* fix(providers): validate initial thinking config
* refactor(providers): centralize thinking capability
* fix(server): order config mutation events
