Ensure AspireStore base path is absolute

[sebastienros]

Description

Add better error message when the AspireStore base path is invalid
c.f. #7511

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
    • Is this introducing a breaking change?
      • Yes
        • Link to aspire-docs issue (please use this breaking-change template):
      • No
        • Link to aspire-docs issue (please use this doc-idea template):
  • No

[eerhardt]

Can we ensure it is rooted when we put it in the AssemblyMetadataAttribute?

https://github.com/dotnet/aspire/blob/0ab5904e229f339035caf9d3e45f590badc7321b/src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets#L245-L252

[sebastienros]

Right. I realized when the poster explained their value was obj/. I assume in the aspire solution we rewrite it for the artifacts folder. But the default for external project is obj/ as per documentation.

Does the change seem right? I tested it with different values.

[eerhardt]

I think this looks good.

[sebastienros]

/cc @DamianEdwards in case you also need paths to be absolute in the code you did