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/cmd/util/util.go

@@ -25,11 +25,7 @@ func Prompt(ctx *lmcli.Context, messages []conversation.Message, callback func(c
 	}
 	p := modelConfig.Client
 
-	params := provider.RequestParameters{
-		Model:       modelConfig.Model,
-		MaxTokens:   modelConfig.MaxTokens,
-		Temperature: modelConfig.Temperature,
-	}
+	params := provider.NewRequestParameters(*modelConfig)
 
 	system := ctx.DefaultSystemPrompt()