[Fusion] Add AddHttpResponseFormatter and AddSocketSessionInterceptor

[tobias-tengler]

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[Copilot]

Pull request overview

This PR extends the Fusion ASP.NET Core gateway builder API to allow customizing outbound HTTP behavior and WebSocket session handling, and adds tests to validate the new DI extension methods.

Changes:

• Add AddSocketSessionInterceptor overloads to configure GraphQL-over-WebSocket session interception for Fusion gateways.
• Add AddHttpResponseFormatter overloads to configure/replace the gateway’s IHttpResponseFormatter (including an indented JSON option).
• Add new Fusion ASP.NET Core tests (including WebSocket coverage) and required test-project references/utilities.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/HotChocolate/Fusion/src/Fusion.AspNetCore/DependencyInjection/AspNetCoreFusionGatewayBuilderExtensions.HttpRequestInterceptor.cs	Adds new gateway builder extension methods for socket session interception and HTTP response formatting.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/GatewayBuilderInterceptorTests.cs	New tests verifying interceptors and HTTP response formatter registration behavior (HTTP + WebSocket).
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/FusionTestBase.cs	Exposes TestServer.CreateWebSocketClient() via the test Gateway wrapper.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/HotChocolate.Fusion.AspNetCore.Tests.csproj	Adds project reference needed for WebSocket client test support.

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