Fix serialization issue with enum when building capabilities

[IEvangelist]

Description

Add JsonStringEnumConverter to JSON serialization options in AtsMarshaller and CapabilityDispatcher.

Contributes to #14772

Resolves this following error:

❌ Uncaught Exception: The JSON value could not be converted to System.Nullable`1[[Aspire.Hosting.ApplicationModel.IconVariant]]. Path: $.iconVariant | LineNumber: 0 |        
BytePositionInLine: 54.
CapabilityError: The JSON value could not be converted to System.Nullable`1[[Aspire.Hosting.ApplicationModel.IconVariant]]. Path: $.iconVariant | LineNumber: 0 |
BytePositionInLine: 54.
    at AspireClient.invokeCapability (D:\GitHub\TsApp\.modules\transport.ts:536:23)
    at async PostgresDatabaseResource._withCommandInternal (D:\GitHub\TsApp\.modules\aspire.ts:4648:24)
    at async <anonymous> (D:\GitHub\TsApp\apphost.ts:26:1)

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.dev issue:
      • New issue
  • 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 -- 14784

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 14784"

[Copilot]

Pull request overview

Adds JsonStringEnumConverter to the JSON serialization options used by ATS marshalling and capability dispatching to ensure enums can round-trip as strings (fixing enum deserialization failures like iconVariant).

Changes:

• Add JsonStringEnumConverter to CapabilityDispatcher JSON options for DTO parsing.
• Add JsonStringEnumConverter to AtsMarshaller JSON options for DTO marshal/unmarshal.
• Add unit tests covering enum string serialization/deserialization behaviors (including case-insensitive reads).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/Aspire.Hosting.RemoteHost.Tests/CapabilityDispatcherTests.cs	Adds GetDto tests validating enum properties deserialize from string values.
tests/Aspire.Hosting.RemoteHost.Tests/AtsMarshallerTests.cs	Adds tests asserting enum string converter is present and enums round-trip as strings.
src/Aspire.Hosting.RemoteHost/Ats/CapabilityDispatcher.cs	Configures dispatcher JSON options to include JsonStringEnumConverter.
src/Aspire.Hosting.RemoteHost/Ats/AtsMarshaller.cs	Configures marshaller JSON options to include JsonStringEnumConverter.