Fix AspireHostLauncher not launching the Aspire host

[karolz-ms]

No description provided.

[Copilot]

Pull request overview

This PR fixes AspireHostLauncher so the Aspire host project is actually launched (by providing MainProjectOptions to the watcher), and strengthens tests to detect regressions.

Changes:

• Pass host ProjectOptions into DotNetWatchContext.MainProjectOptions when launching the Aspire watcher.
• Adjust AspireHostLauncher/tests to use the new project-options flow (GetHostProjectOptions wrapper for testing).
• Update the Aspire launcher end-to-end test to assert the host app outputs Started.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/Microsoft.DotNet.HotReload.Watch.Aspire.Tests/AspireLauncherTests.cs	Adds a runtime-output assertion (Started) to confirm the host app is actually launched.
test/Microsoft.DotNet.HotReload.Watch.Aspire.Tests/AspireHostLauncherTests.cs	Updates tests to call the new host project-options accessor.
src/Dotnet.Watch/Watch.Aspire/Server/AspireWatcherLauncher.cs	Wires MainProjectOptions from an overridable GetProjectOptions() to enable main-project launching.
src/Dotnet.Watch/Watch.Aspire/Host/AspireHostLauncher.cs	Implements GetProjectOptions() override so the host launch supplies MainProjectOptions.

Comments suppressed due to low confidence (1)

test/Microsoft.DotNet.HotReload.Watch.Aspire.Tests/AspireHostLauncherTests.cs:51

• GetHostProjectOptions() returns a non-null ProjectOptions, so the null-forgiving operator (!) is redundant here (and can hide nullability issues if the signature changes later). Consider removing the ! (and similarly in the other occurrences in this file once the underlying return type is non-null).

        var options = launcher.GetHostProjectOptions()!;

        AssertCommonProperties(options, launcher);
        Assert.False(options.LaunchProfileName.HasValue);
        AssertEx.SequenceEqual(["--project", "myapp.csproj", "--no-launch-profile"], options.CommandArguments);

[tmat]

/ba-g Unrelated issues.

Copilot analysis:

The CI has two distinct categories of test failures, all across 5 legs (Windows/Linux/macOS):

1. ✅ Known Issue — RunFileTests csc-only binary log warning

Tracked by #53869 (#53869) ([Known Build Error]). Affects
RunFileTests_BuildCommands.LaunchProfile, RunFileTests_BuildOptions.WorkingDirectory,
RunFileTests_CscOnlyAndApi.CscOnly. Reproduces on main.

2. ❌ NOT tracked — "Using launch settings" stderr failures

These tests fail because dotnet run now outputs "Using launch settings from ...launchSettings.json..." to stderr,
but the tests assert empty/no stderr:

• GivenDotnetRunRunsVbproj.ItUsesLaunchProfileOfTheSpecifiedName("Second")
• GivenDotnetRunRunsVbproj.ItUsesLaunchProfileOfTheSpecifiedName("sEcoND")
• GivenDotnetRunRunsVbproj.ItDefaultsToTheFirstUsableLaunchProfile
• RunCommandTests.EnvironmentVariableExpansion_Project

These are not tracked by any existing [Known Build Error] issue in dotnet/sdk. They appear to be a pre-existing
issue on main (not caused by PR #53877's Aspire watch changes), since the "Using launch settings" message is
unrelated to the 4 files changed in this PR.

[tmat]

/backport to release/10.0.3xx

[github-actions]

Started backporting to release/10.0.3xx (link to workflow run)