Session.SetModel shouldn't accept a variadic option (#904)

* Session.SetModel shouldn't accept a variadic option

* cleanup

Author: qmuntal

go/internal/e2e/rpc_test.go

@@ -202,8 +202,7 @@ func TestSessionRpc(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Failed to create session: %v", err)
 		}
-
-		if err := session.SetModel(t.Context(), "gpt-4.1", copilot.SetModelOptions{ReasoningEffort: "high"}); err != nil {
+		if err := session.SetModel(t.Context(), "gpt-4.1", &copilot.SetModelOptions{ReasoningEffort: copilot.String("high")}); err != nil {
 			t.Fatalf("SetModel returned error: %v", err)
 		}
 	})

go/internal/e2e/session_test.go

@@ -966,7 +966,7 @@ func TestSetModelWithReasoningEffort(t *testing.T) {
 		}
 	})
 
-	if err := session.SetModel(t.Context(), "gpt-4.1", copilot.SetModelOptions{ReasoningEffort: "high"}); err != nil {
+	if err := session.SetModel(t.Context(), "gpt-4.1", &copilot.SetModelOptions{ReasoningEffort: copilot.String("high")}); err != nil {
 		t.Fatalf("SetModel returned error: %v", err)
 	}
 

go/session.go

@@ -792,25 +792,24 @@ func (s *Session) Abort(ctx context.Context) error {
 // SetModelOptions configures optional parameters for SetModel.
 type SetModelOptions struct {
 	// ReasoningEffort sets the reasoning effort level for the new model (e.g., "low", "medium", "high", "xhigh").
-	ReasoningEffort string
+	ReasoningEffort *string
 }
 
 // SetModel changes the model for this session.
 // The new model takes effect for the next message. Conversation history is preserved.
 //
 // Example:
 //
-//	if err := session.SetModel(context.Background(), "gpt-4.1"); err != nil {
+//	if err := session.SetModel(context.Background(), "gpt-4.1", nil); err != nil {
 //	    log.Printf("Failed to set model: %v", err)
 //	}
-//	if err := session.SetModel(context.Background(), "claude-sonnet-4.6", SetModelOptions{ReasoningEffort: "high"}); err != nil {
+//	if err := session.SetModel(context.Background(), "claude-sonnet-4.6", &SetModelOptions{ReasoningEffort: new("high")}); err != nil {
 //	    log.Printf("Failed to set model: %v", err)
 //	}
-func (s *Session) SetModel(ctx context.Context, model string, opts ...SetModelOptions) error {
+func (s *Session) SetModel(ctx context.Context, model string, opts *SetModelOptions) error {
 	params := &rpc.SessionModelSwitchToParams{ModelID: model}
-	if len(opts) > 0 && opts[0].ReasoningEffort != "" {
-		re := opts[0].ReasoningEffort
-		params.ReasoningEffort = &re
+	if opts != nil {
+		params.ReasoningEffort = opts.ReasoningEffort
 	}
 	_, err := s.RPC.Model.SwitchTo(ctx, params)
 	if err != nil {