[release/13.0] EndpointReference evaluation should wait on missing AllocatedEndpoint (#13074)

[karolz-ms]

Description

A fix to restore proper EndpointReference resolution behavior, which should wait on missing AllocatedEndpoint instead of resolving as empty value. This bug can break existing applications and tests.

Fixes #12823

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-docs issue (consider using one of the following templates):
      • New (or update) doc-idea template
      • New breaking-change template
      • New diagnostic template
  • 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/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 13076

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 13076"

[Copilot]

Pull Request Overview

This PR fixes a critical bug where EndpointReference evaluation would incorrectly resolve as an empty value instead of waiting for a missing AllocatedEndpoint. This change restores the proper asynchronous waiting behavior, preventing potential failures in applications and tests that depend on endpoint resolution.

Key Changes:

• Modified EndpointReferenceExpression.GetValueAsync to create and wait on a ValueSnapshot<AllocatedEndpoint> when no snapshot exists for the requested network context, instead of immediately returning null
• Added thread-safe handling using TryAdd to prevent race conditions when multiple threads attempt to create snapshots for the same network
• Included comprehensive test coverage to verify the fix works correctly

Reviewed Changes

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

File	Description
src/Aspire.Hosting/ApplicationModel/EndpointReference.cs	Fixed the endpoint resolution logic to create and wait on a ValueSnapshot instead of returning null when no AllocatedEndpoint exists for the requested network context
tests/Aspire.Hosting.Tests/ExpressionResolverTests.cs	Added test ExpressionResolutionShouldWaitOnMissingAllocatedEndpoint to verify the fix, along with a minimal TestHostResource helper class