Add user picked model to be used as a default for open router provider when generating comments and thread summary

[zapp88]

Closes #37525

By default, thread summary uses default_fast_model (if set), otherwise default_model, which resolves to openrouter/auto for openrouter provider. This may cause the summary to be generated by a different model than the one used by the agent, potentially leading — in cases such as Claude Opus 4.5 — to summary costs exceeding main agent execution costs.
The current logic in registry.rs prioritizes default_fast_model over default_model, which overrides the user-selected model (assigned only to default_model). Setting default_fast_model = None for the OpenRouter provider preserves the fallback to openrouter/auto when no model is chosen, while respecting the user's explicit model selection when one is provided.

    pub fn set_default_model(&mut self, model: Option<ConfiguredModel>, cx: &mut Context<Self>) {
        match (self.default_model.as_ref(), model.as_ref()) {
            (Some(old), Some(new)) if old.is_same_as(new) => {}
            (None, None) => {}
            _ => cx.emit(Event::DefaultModelChanged),
        }
        self.default_fast_model = maybe!({
            let provider = &model.as_ref()?.provider;
            let fast_model = provider.default_fast_model(cx)?;
            Some(ConfiguredModel {
                provider: provider.clone(),
                model: fast_model,
            })
        }); // This sets default fast model (in our case openrouter/auto)
        self.default_model = model;  //This sets default_model to user selected model
    }

And latter on :

    pub fn thread_summary_model(&self) -> Option<ConfiguredModel> {
        #[cfg(debug_assertions)]
        if std::env::var("ZED_SIMULATE_NO_LLM_PROVIDER").is_ok() {
            return None;
        }

        self.thread_summary_model
            .clone()
            .or_else(|| self.default_fast_model.clone()) // We pick fast_model over default model here
            .or_else(|| self.default_model.clone())
    }

Which results in user choice being ignored.

Proposed behavior:

Use the model explicitly selected by the user in Zed agent configuration.
If no model is specified, fall back to the configured default.

The resolution is to set in : provider/open_router.rs

fn default_fast_model(&self, _cx: &App) -> Option<Arc<dyn LanguageModel>> {
     None
}

This will have a consequence of default_fast_model not being provided and falling back to user choice - but once the fast model is set via for example a configuration property - the default_fast_model is picked over default_model

Release Notes:

• open_router: Use user's default model when comments and thread summary