Re-enable Microsoft.Extensions.Hosting FunctionalTests/TestApp shutdown coverage

[Copilot]

Microsoft.Extensions.Hosting functional shutdown tests were effectively dead code: they were globally suppressed and pointed at an empty app path, so they never executed meaningful TestApp coverage.
This updates the tests to use the in-repo TestApp again and removes the suppression that prevented them from running.

• FunctionalTests/TestApp wiring

  • Replaced the disabled applicationPath = string.Empty path with repository-root discovery logic for tests/TestApp.
  • Set DeploymentParameters.ApplicationName to Microsoft.Extensions.Hosting.TestApp so deploy/run targets the correct published assembly.

• Re-enable test execution

  • Removed IgnoreForCI from Microsoft.Extensions.Hosting.Functional.Tests.csproj.
  • Removed ActiveIssue assembly-level suppression from FunctionalTests/Properties/AssemblyInfo.cs.

var deploymentParameters = new DeploymentParameters(
    applicationPath,
    RuntimeFlavor.CoreClr,
    RuntimeArchitecture.x64)
{
    ApplicationName = "Microsoft.Extensions.Hosting.TestApp",
    TargetFramework = $"net{version.Major}.{version.Minor}",
    ApplicationType = ApplicationType.Portable,
    PublishApplicationBeforeDeployment = true,
    StatusMessagesEnabled = false
};

Fixes #34090