[docs] Update multi-language integration guide for ATS-first API design

[aspire-repo-bot]

Documents changes from microsoft/aspire#16403

Author: @sebastienros

Targeting main — falling back to main because release/13.3 (resolved from the source PR milestone 13.3) does not exist on microsoft/aspire.dev.

Why this PR is needed

PR microsoft/aspire#16403 adopts an ATS-first API design for Aspire.Hosting exported capabilities. This introduces two user-facing changes that the current docs don't cover:

1. Getter-only properties → async methods in TypeScript. When a C# property is getter-only (no setter, and not a mutable-collection type), the code generator now emits it as an async method in the TypeScript SDK (property(): Promise<T>). Previously it was a readonly getter. This is a breaking change for any TypeScript AppHost code that uses ATS-generated properties. The current multi-language-integration-authoring.mdx page says "each public property becomes a capability" but doesn't explain this distinction.

2. New ATS-first callback context pattern. The PR introduces EnvironmentEditor, CommandLineArgsEditor, and ResourceUrlsEditor facade types, and exports withEnvironmentCallback, withArgsCallback, and withUrls callback APIs. Integration authors following the old pattern of ExposeProperties = true on callback context classes need to know the new recommended approach: use individual [AspireExport] attributes and wrap mutable state in editor classes.

What was changed

• Updated src/frontend/src/content/docs/extensibility/multi-language-integration-authoring.mdx:
  • Expanded the "Export resource types" section with individual-property export guidance alongside ExposeProperties = true
  • Added a new subsection "How getter-only properties appear in TypeScript" explaining the async-method vs readonly-getter distinction with before/after code examples
  • Added a new top-level section "Callback context types and the ATS-first editor pattern" covering:
    • The three-rule design checklist for callback context types
    • How to define an editor class ([AspireExport] on the class + [AspireExport] on its methods)
    • How to define the context class with individual [AspireExport] attributes on the properties TypeScript callers need
    • How to export the callback extension method and what the generated TypeScript looks like

Files modified

File	Action
src/frontend/src/content/docs/extensibility/multi-language-integration-authoring.mdx	Updated

This draft PR needs human review before merging — in particular, the C# code examples for the editor class and callback context type should be verified against the actual types shipped in 13.3.

Note

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