Fix some argument validation in C#

[stephentoub]

Also clean up usings

[Copilot]

Pull request overview

This PR tightens .NET SDK argument validation and cleans up unused/reordered using directives across source and test files.

Changes:

• Adds null argument checks to selected CopilotClient, CopilotSession, and SessionFsProvider methods.
• Removes unused usings and normalizes using ordering across .NET tests and source files.

Show a summary per file

File	Description
dotnet/src/Client.cs	Adds null validation for public client/session lifecycle APIs and cleans usings.
dotnet/src/Session.cs	Adds null validation for message, handler, model, and log APIs.
dotnet/src/SessionFsProvider.cs	Adds null validation for session filesystem request handlers.
dotnet/src/Types.cs	Reorders usings.
dotnet/src/JsonRpc.cs	Removes unused using and reorders usings.
dotnet/src/Polyfills/Utf8.cs	Removes unused using.
dotnet/src/Polyfills/DownlevelExtensions.cs	Removes unused usings.
dotnet/test/ConnectionTokenTests.cs	Removes unused using.
dotnet/test/Harness/E2ETestBase.cs	Reorders usings.
dotnet/test/Harness/E2ETestContext.cs	Reorders usings.
dotnet/test/Unit/CloneTests.cs	Removes unused using.
dotnet/test/Unit/JsonRpcTests.cs	Removes unused using.
dotnet/test/Unit/SerializationTests.cs	Removes unused using.
dotnet/test/Unit/SessionEventSerializationTests.cs	Removes unused using.
dotnet/test/E2E/AbortE2ETests.cs	Removes/reorders usings.
dotnet/test/E2E/BuiltinToolsE2ETests.cs	Removes unused using.
dotnet/test/E2E/ClientOptionsE2ETests.cs	Removes unused using.
dotnet/test/E2E/CommandsE2ETests.cs	Removes unused using.
dotnet/test/E2E/ElicitationE2ETests.cs	Removes unused using.
dotnet/test/E2E/ErrorResilienceE2ETests.cs	Removes unused using.
dotnet/test/E2E/HookLifecycleAndOutputE2ETests.cs	Removes unused using.
dotnet/test/E2E/MultiClientE2ETests.cs	Removes/reorders usings.
dotnet/test/E2E/MultiTurnE2ETests.cs	Removes unused using.
dotnet/test/E2E/PendingWorkResumeE2ETests.cs	Reorders usings.
dotnet/test/E2E/RpcEventSideEffectsE2ETests.cs	Reorders usings.
dotnet/test/E2E/RpcMcpConfigE2ETests.cs	Reorders usings.
dotnet/test/E2E/RpcServerE2ETests.cs	Removes unused using.
dotnet/test/E2E/RpcSessionStateE2ETests.cs	Reorders usings.
dotnet/test/E2E/RpcShellAndFleetE2ETests.cs	Removes unused using.
dotnet/test/E2E/SessionConfigE2ETests.cs	Removes/reorders usings.
dotnet/test/E2E/SessionE2ETests.cs	Reorders usings.
dotnet/test/E2E/StreamingFidelityE2ETests.cs	Removes unused using.
dotnet/test/E2E/SuspendE2ETests.cs	Removes/reorders usings.
dotnet/test/E2E/ToolsE2ETests.cs	Removes unused using.

Copilot's findings

• Files reviewed: 34/34 changed files
• Comments generated: 0

[github-actions]

Cross-SDK Consistency Review ✅

This PR adds null argument validation (ArgumentNullException.ThrowIfNull) to public API methods in the .NET SDK. All 34 changed files are within dotnet/ — no changes to Node.js, Python, Go, or Rust.

Cross-language parity check:

Method	Go	Python	Node.js
CreateSession(config)	✅ Already checks config == nil	✅ on_permission_request is a required kwarg	✅ TypeScript non-nullable type
ResumeSession(sessionId, config)	✅ Already checks config == nil	✅ session_id: str (required, not str | None)	✅ TypeScript non-nullable type
DeleteSession(sessionId)	N/A — Go string is a value type (can't be nil)	N/A — Python str is required	N/A — TypeScript non-nullable
On(handler)	Go function types can be nil (no guard, but Go-idiomatic)	Not applicable (callable checked by type system)	✅ TypeScript non-nullable
Send(options)	Takes MessageOptions by value (can't be nil)	Named kwargs pattern	✅ TypeScript non-nullable
SetModel(model) / Log(message)	Go string value type	Python str required param	✅ TypeScript non-nullable

Summary: The null checks added here are appropriate and .NET-idiomatic. The other SDKs achieve equivalent protection through their own language mechanisms (Go value types for strings, Python required keyword arguments, TypeScript's type system). No cross-SDK parity gaps were identified.

The SessionFsProvider null checks on request parameters are internal interface implementations and don't affect the public API surface.

Generated by SDK Consistency Review Agent for issue #1322 · ● 598.4K · ◷