Add generated projects to solution does not work for `.slnx`. (D396686C-DE0E-4DE6-906D-291CD29FC5DE)

### Product

dotnet CLI (dotnet new)

### Describe The Bug

The "add project to solution" post-action seems to ignore `.slnx` files.

I have a template that was working before we migrated. Now it fails with:

```
Processing post-creation actions...
Unable to determine which solution file to add the reference to.
Post action failed.
Manual instructions: Add generated projects to solution.
```

### To Reproduce

Steps:
1. Create a .NET new template that creates a project.
2. Use the following post-action:

```json
"postActions": [
{
"description": "Add the project to the solution.",
"manualInstructions": [
{
"text": "Add generated projects to solution."
}
],
"actionId": "D396686C-DE0E-4DE6-906D-291CD29FC5DE",
"continueOnError": false
},
```
3. Run this in a directory with an existing `.slnx` file (and no `.sln` file).

### dotnet Info

output
 dotnet --info
.NET SDK:
Version: 9.0.201
Commit: 071aaccdc2
Workload version: 9.0.200-manifests.21502d11
MSBuild version: 17.13.13+1c2026462

Runtime Environment:
OS Name: Mac OS X
OS Version: 15.4
OS Platform: Darwin
RID: osx-arm64
Base Path: /usr/local/share/dotnet/sdk/9.0.201/

.NET workloads installed:
There are no installed workloads to display.
Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.3
Architecture: arm64
Commit: 831d23e561

.NET SDKs installed:
9.0.102 [/usr/local/share/dotnet/sdk]
9.0.200 [/usr/local/share/dotnet/sdk]
9.0.201 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 9.0.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.2 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 9.0.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
/Users/ryan/github.com/infinity-microsoft/picasso/global.json

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

### Visual Studio Version

_No response_

### Additional context

_No response_