Document cross-assembly ATS exports

[sebastienros]

Description

This documents the cross-assembly ATS export behavior behind polyglot AppHost type generation so issue #14884 has a concrete explanation in the existing ATS spec. Reviewers were missing guidance on what happens when one assembly exports a type and another assembly contributes capabilities that use it.

The update keeps the change scoped to the existing spec by expanding docs/specs/polyglot-apphost.md with a focused cross-assembly exports subsection and by adding a small related cleanup in ThirdPartyAtsAttributes.md. The new text explains when to use assembly-level AspireExport(typeof(T)), how ATS type IDs are derived from the exported CLR type, how multi-assembly scanning merges exported types and capabilities, and why duplicate capability IDs are still errors.

Fixes #14884

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire.dev issue:
      • New issue
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 15810

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 15810"

[Copilot]

Pull request overview

Updates the polyglot AppHost ATS specification to explicitly document how type exports and capability scanning behave when types and capabilities are split across multiple assemblies (addressing #14884), and links that guidance from the third-party ATS attribute copy instructions.

Changes:

• Documented cross-assembly ATS exports in the polyglot AppHost spec (type ID derivation, multi-assembly scan merging, and duplicate capability ID errors).
• Clarified that ATS type IDs are derived from {AssemblyName}/{FullTypeName} in the third-party attributes documentation and added a pointer to the spec section.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Aspire.Hosting/Ats/ThirdPartyAtsAttributes.md	Updates type ID wording to use full type name and links to the spec’s cross-assembly export behavior section.
docs/specs/polyglot-apphost.md	Adds a focused “Cross-Assembly Type Exports” subsection explaining scanning/merging behavior and type ID source.