Improve setModel API: variadic Go opts, reuse TS ReasoningEffort type, fix .NET overloads

- Go: use variadic ...SetModelOptions for backward compatibility
- TypeScript: reuse ReasoningEffort type instead of inline union
- .NET: remove redundant default on reasoningEffort param, make CancellationToken optional in both overloads
- Revert unrelated dotnet/test/RpcTests.cs change

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/Session.cs

@@ -663,23 +663,23 @@ await InvokeRpcAsync<object>(
     /// The new model takes effect for the next message. Conversation history is preserved.
     /// </summary>
     /// <param name="model">Model ID to switch to (e.g., "gpt-4.1").</param>
-    /// <param name="reasoningEffort">Optional reasoning effort level (e.g., "low", "medium", "high", "xhigh").</param>
+    /// <param name="reasoningEffort">Reasoning effort level (e.g., "low", "medium", "high", "xhigh").</param>
     /// <param name="cancellationToken">Optional cancellation token.</param>
     /// <example>
     /// <code>
     /// await session.SetModelAsync("gpt-4.1");
     /// await session.SetModelAsync("claude-sonnet-4.6", SessionModelSwitchToRequestReasoningEffort.High);
     /// </code>
     /// </example>
-    public async Task SetModelAsync(string model, SessionModelSwitchToRequestReasoningEffort? reasoningEffort = null, CancellationToken cancellationToken = default)
+    public async Task SetModelAsync(string model, SessionModelSwitchToRequestReasoningEffort? reasoningEffort, CancellationToken cancellationToken = default)
     {
         await Rpc.Model.SwitchToAsync(model, reasoningEffort, cancellationToken);
     }
 
     /// <summary>
-    /// Changes the model for this session (backward-compatible overload).
+    /// Changes the model for this session.
     /// </summary>
-    public Task SetModelAsync(string model, CancellationToken cancellationToken)
+    public Task SetModelAsync(string model, CancellationToken cancellationToken = default)
     {
         return SetModelAsync(model, reasoningEffort: null, cancellationToken);
     }

dotnet/test/RpcTests.cs

@@ -73,7 +73,7 @@ public async Task Should_Call_Session_Rpc_Model_SwitchTo()
         Assert.NotNull(before.ModelId);
 
         // Switch to a different model
-        var result = await session.Rpc.Model.SwitchToAsync(modelId: "gpt-4.1", reasoningEffort: null);
+        var result = await session.Rpc.Model.SwitchToAsync(modelId: "gpt-4.1");
         Assert.Equal("gpt-4.1", result.ModelId);
 
         // Verify the switch persisted

go/internal/e2e/rpc_test.go

@@ -201,7 +201,7 @@ func TestSessionRpc(t *testing.T) {
 			t.Fatalf("Failed to create session: %v", err)
 		}
 
-		if err := session.SetModel(t.Context(), "gpt-4.1", nil); err != nil {
+		if err := session.SetModel(t.Context(), "gpt-4.1"); err != nil {
 			t.Fatalf("SetModel returned error: %v", err)
 		}
 	})

go/session.go

@@ -693,20 +693,19 @@ type SetModelOptions struct {
 
 // SetModel changes the model for this session.
 // The new model takes effect for the next message. Conversation history is preserved.
-// Pass nil for opts if no additional options are needed.
 //
 // Example:
 //
-//	if err := session.SetModel(context.Background(), "gpt-4.1", nil); err != nil {
+//	if err := session.SetModel(context.Background(), "gpt-4.1"); 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: "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 opts != nil && opts.ReasoningEffort != "" {
-		re := opts.ReasoningEffort
+	if len(opts) > 0 && opts[0].ReasoningEffort != "" {
+		re := opts[0].ReasoningEffort
 		params.ReasoningEffort = &re
 	}
 	_, err := s.RPC.Model.SwitchTo(ctx, params)

nodejs/src/session.ts

@@ -15,6 +15,7 @@ import type {
     PermissionHandler,
     PermissionRequest,
     PermissionRequestResult,
+    ReasoningEffort,
     SessionEvent,
     SessionEventHandler,
     SessionEventPayload,
@@ -694,7 +695,7 @@ export class CopilotSession {
      */
     async setModel(
         model: string,
-        options?: { reasoningEffort?: "low" | "medium" | "high" | "xhigh" }
+        options?: { reasoningEffort?: ReasoningEffort }
     ): Promise<void> {
         await this.rpc.model.switchTo({ modelId: model, ...options });
     }