Refactor a common TempDirectory test class

[eerhardt]

DockerComposePublisherTests was using File.Exists and File.Delete, which don't work for directories.

[Copilot]

Pull Request Overview

This PR refactors the temporary directory handling in tests by replacing several ad-hoc local TempDirectory implementations with a shared TempDirectory class.

• Introduces tests/Shared/TempDirectory.cs with a common implementation for creating and cleaning up temporary directories.
• Updates multiple test files to use the new TempDirectory instead of manually creating and deleting directories.
• Removes redundant local implementations of TempDirectory in Kubernetes and Docker tests.

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Shared/TempDirectory.cs	New shared TempDirectory class to create and clean up temp directories for tests.
tests/Aspire.Hosting.Tests/PublishAsDockerfileTests.cs	Changed usage of temporary directory from Directory.CreateTempSubdirectory() to the shared TempDirectory.
tests/Aspire.Hosting.Tests/MSBuildTests.cs	Updated to use TempDirectory and removed redundant cleanup code.
tests/Aspire.Hosting.PostgreSQL.Tests/PostgresFunctionalTests.cs	Refactored temporary directory handling to use TempDirectory.
tests/Aspire.Hosting.PostgreSQL.Tests/AddPostgresTests.cs	Updated to use TempDirectory for temporary store path configuration.
tests/Aspire.Hosting.MySql.Tests/MySqlFunctionalTests.cs	Refactored TempDirectory usage for persistent lifetime tests.
tests/Aspire.Hosting.MySql.Tests/AddMySqlTests.cs	Updated temporary store initialization to use TempDirectory.
tests/Aspire.Hosting.Kubernetes.Tests/KubernetesPublisherTests.cs	Removed local TempDirectory class in favor of shared implementation.
tests/Aspire.Hosting.Docker.Tests/DockerComposePublisherTests.cs	Removed obsolete local TempDirectory that incorrectly used file deletion logic.