Fix a regression in endpoint resolution in environment variables

[davidfowl]

Description

• Endpoints wrapped in connection strings were throwing during the preprocess pass of the evalution. Avoid throwing during that pass
• Added tests for this scenario

Fixes #8596

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
• 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]

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

Comments suppressed due to low confidence (1)

src/Aspire.Hosting/ApplicationModel/ExpressionResolver.cs:163

• Consider adding tests for scenarios when Preprocess is true to ensure that missing connection strings do not trigger an exception during pre-processing.

if (!Preprocess && string.IsNullOrEmpty(value.Value) && !cs.Optional)

[davidfowl]

/backport to release/9.2

[github-actions]

Started backporting to release/9.2: https://github.com/dotnet/aspire/actions/runs/14323470219

[danmoseley]

hey @afscrome thanks for finding this and other last minute issues. really helps.