Fix: TextWriterFluidOutput.Dispose() requires force sync (#919)

[sebastienros]

Summary of Changes

This PR adds IAsyncDisposable support to TextWriterFluidOutput and updates all usages to use await using instead of using to ensure fully asynchronous disposal.

Key changes:

• Added IAsyncDisposable interface implementation to TextWriterFluidOutput with DisposeAsync() method
• Updated all callers to use await using pattern:
  • FluidRendering.cs (MVC View Engine)
  • FluidViewRenderer.cs (View Engine - both RenderViewAsync and RenderPartialAsync)
  • FluidOutputExtensions.cs (3 extension methods)
  • FluidTemplateExtensions.cs
  • ActionViewResult.cs (Minimal APIs)
• Added comprehensive unit tests (TextWriterFluidOutputTests.cs - 1073 lines)
• Added integration tests (FluidOutputIntegrationTests.cs - 498 lines)

Motivation and Context

Fixes #919

ASP.NET Core disables synchronous I/O by default (IHttpBodyControlFeature.AllowSynchronousIO = false). The v3.0.0-beta4 release introduced TextWriterFluidOutput which used synchronous Dispose() that internally called FlushCore() with synchronous writes. This caused System.InvalidOperationException: Synchronous operations are disallowed errors when rendering Liquid templates in ASP.NET Core applications, particularly when response compression (e.g., Brotli) was enabled.

How the Changes Were Tested

• Added TextWriterFluidOutputTests.cs with comprehensive unit tests covering:

  • DisposeAsync() behavior and buffer cleanup
  • Async flushing and disposal patterns
  • Leave-open writer scenarios
  • Edge cases around buffer management

• Added FluidOutputIntegrationTests.cs with integration tests covering:

  • End-to-end async rendering scenarios
  • Verification that async disposal path is used correctly

Screenshots

N/A - Internal API change, no UI impact.

Checklist

• Code compiles without errors
• Implemented IAsyncDisposable on TextWriterFluidOutput
• Updated all using statements to await using for async disposal
• Added unit tests for async disposal
• Added integration tests
• Maintains backward compatibility (synchronous Dispose() still available)
• Cross-platform support via #if NETSTANDARD2_0 conditional compilation