[release/13.4] Fix TypeScript AppHost async callback deadlock

[aspire-repo-bot]

Backport of #17575 to release/13.4

/cc @sebastienros @IEvangelist

Customer Impact

Testing

Risk

Regression?

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[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 -- 17689

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 17689"

[aspire-repo-bot]

✅ No documentation update needed.

docs_optional → bug_fix_restores_documented_behavior

No gating signals fired (signal_count = 0, recommendation = docs_optional).

The only source change is a single-line fix in src/Aspire.Hosting/DistributedApplication.cs adding RunSyncOnBackgroundThread = true to the [AspireExport("run")] attribute, which resolves a deadlock when TypeScript AppHosts use async callbacks via lazy IOptions.Configure during BeforeStartEvent. The documented/expected behavior — that await builder.build().run() completes successfully — is restored by this fix. The second changed file is a regression test under tests/. No new public API, CLI flags, configuration keys, or user-facing options were introduced.