Throw if container runtime is unhealthy during image build

[captainsafia]

Closes #10156.

Before:

After:

[Copilot]

Pull Request Overview

This PR ensures the container image build step fails fast with a clear exception when the container runtime is not running or is unhealthy, and adds a corresponding unit test.

• Changed the builder to throw an InvalidOperationException instead of silently returning when the runtime check fails.
• Added a test (BuildImageAsync_ThrowsInvalidOperationException_WhenDockerRuntimeNotAvailable) to verify both the exception and log output in an unhealthy-runtime scenario.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/Aspire.Hosting.Tests/Publishing/ResourceContainerImageBuilderTests.cs	Added test for unhealthy container runtime to assert exception and log
src/Aspire.Hosting/Publishing/ResourceContainerImageBuilder.cs	Switched from return to throwing an exception on runtime failure

Comments suppressed due to low confidence (1)

tests/Aspire.Hosting.Tests/Publishing/ResourceContainerImageBuilderTests.cs:386

• [nitpick] The test method name references BuildImageAsync but calls BuildImagesAsync; consider renaming to BuildImagesAsync_ThrowsInvalidOperationException_WhenDockerRuntimeNotAvailable for consistency.

    public async Task BuildImageAsync_ThrowsInvalidOperationException_WhenDockerRuntimeNotAvailable()

[captainsafia]

/backport to release/9.4

[github-actions]

Started backporting to release/9.4: https://github.com/dotnet/aspire/actions/runs/16279438927