[Ollama] Fix container runtime args for nvidia GPU on podman

[MatsM16]

Closes #983

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

[Copilot]

Pull Request Overview

This PR fixes Nvidia GPU support for Podman container runtime by detecting the container runtime and using the appropriate flags (--device nvidia.com/gpu=all for Podman vs --gpus all for Docker).

Key Changes:

• Extracted Nvidia GPU configuration into a separate WithNvidiaGPUSupport method that checks the container runtime
• Added GetContainerRuntime helper method to read container runtime from configuration (supports both ASPIRE_CONTAINER_RUNTIME and legacy DOTNET_ASPIRE_CONTAINER_RUNTIME)
• Refactored tests to use helper methods (AssertSingleResource and AssertContainerRuntimeArgs) to reduce duplication

Reviewed Changes

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

File	Description
src/CommunityToolkit.Aspire.Hosting.Ollama/OllamaResourceBuilderExtensions.cs	Adds runtime detection logic to apply correct GPU arguments for Podman vs Docker, introduces WithNvidiaGPUSupport and GetContainerRuntime helper methods
tests/CommunityToolkit.Aspire.Hosting.Ollama.Tests/AddOllamaTests.cs	Refactors GPU tests with helper methods, adds tests for Podman with both new and legacy configuration keys