Improve path prompt for aspire new command.

[mitchdenny]

This PR improves the way that we prompt for the output path in the aspire new command. Previously we howed the absolution path as a default. Now we show a relative path which the user can accept or override with a new relative path or a new absolute path:

aspire-new-path-fix.mp4

[Copilot]

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

Comments suppressed due to low confidence (2)

src/Aspire.Cli/DotNetCliRunner.cs:26

• Ensure that appending projectFile.FullName to the msbuild arguments is intended and that projectFile is valid in all execution paths.

string[] cliArgs = ["msbuild", "-getproperty:IsAspireHost,AspireHostingSDKVersion", projectFile.FullName];

src/Aspire.Cli/Commands/NewCommand.cs:95

• Verify that using pathAppendage directly as the default output path meets the requirement for a relative path and does not inadvertently impact downstream file operations that might expect an absolute path.

defaultValue: pathAppendage ?? ".",

[mitchdenny]

/backport to release/9.2

[github-actions]

Started backporting to release/9.2: https://github.com/dotnet/aspire/actions/runs/14257138490