Use STJ source generation in Cli backchannel

[eerhardt]

Description

This sets us up for being AOT compatible once the build system is able to build for AOT on all platforms.

Contributes to #8677

FYI @AArnott

Checklist

• Is this feature complete?
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • No
• Did you add public API?
  • No
• Does the change make any security assumptions or guarantees?
  • No
• Does the change require an update in our Aspire docs?
  • No

[Copilot]

Pull Request Overview

This PR integrates System.Text.Json source generation into the CLI backchannel to prepare for future AOT compatibility.

• Extracts formatter creation into CreateMessageFormatter() and applies source-generated JSON metadata
• Suppresses reflection/AOT analysis warnings for the new formatter
• Disables reflection-based JSON by default in the CLI project file and pins an alpha StreamJsonRpc version

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Aspire.Cli/Backchannel/AppHostBackchannel.cs	Replaced inline SystemTextJsonFormatter with CreateMessageFormatter(), added suppression attributes, and introduced SourceGenerationContext
src/Aspire.Cli/Aspire.Cli.csproj	Set JsonSerializerIsReflectionEnabledByDefault to false, commented out PublishAot, and updated StreamJsonRpc package reference

Comments suppressed due to low confidence (3)

src/Aspire.Cli/Backchannel/AppHostBackchannel.cs:153

• Consider adding an XML doc comment on CreateMessageFormatter to explain its purpose and the rationale for the suppression attributes, improving maintainability and discoverability.

    private static SystemTextJsonFormatter CreateMessageFormatter()

src/Aspire.Cli/Backchannel/AppHostBackchannel.cs:198

• Add unit or integration tests that serialize and deserialize expected backchannel messages using this SourceGenerationContext to guard against future regressions in the AOT scenario.

[JsonSerializable(typeof(string[]))]

src/Aspire.Cli/Backchannel/AppHostBackchannel.cs:206

• [nitpick] The name SourceGenerationContext is very generic; consider renaming it to something more descriptive (e.g., CliBackchannelJsonContext) to avoid collisions and clarify its scope.

internal partial class SourceGenerationContext : JsonSerializerContext;