Fix missing collapse on trace detail

[JamesNK]

Description

There was a regression in https://github.com/dotnet/aspire/pull/6261. The children collection on a span view model wasn't populated, which prevented the collapse button from being displayed.

Fixed, and a test added.

Checklist

• Is this feature complete?
  • [x] Yes. Ready to ship.
  • [ ] No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • [x] 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
  • [x] 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
  • [x] 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):
  • [x] No

  Microsoft Reviewers: Open in CodeFlow

[JamesNK]

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