Add support for openrouter reasoning + refactor

Started work to make it possible to pass in per-model reasoning config
Cleaned up how we instantiate RequestParameters (TBD: remove
RequestParameters?)

pkg/tui/model/model.go

@@ -263,11 +263,7 @@ func (a *AppModel) Prompt(
 	}
 	p := modelConfig.Client
 
-	params := provider.RequestParameters{
-		Model:       modelConfig.Model,
-		MaxTokens:   modelConfig.MaxTokens,
-		Temperature: modelConfig.Temperature,
-	}
+	params := provider.NewRequestParameters(*modelConfig)
 
 	if a.Agent != nil {
 		params.Toolbox = a.Agent.Toolbox