Fix .NET caching prerequisites and test artifact workflow - #46336

Why:

Two examples in "Building and testing .NET" can prevent readers from running the documented workflows:

• The test artifact example nests steps under strategy, which is not a valid GitHub Actions job structure.
• The caching example enables cache: true without explaining that a committed NuGet lock file must already be available. setup-dotnet fails when it cannot find a matching lock file.

What's being changed (if available, include any code snippets, screenshots, or gifs):

• Move the artifact example's five existing steps to the job level, preserving their commands and the always() upload condition.
• Explain how to generate and commit packages.lock.json before enabling caching.
• Show cache-dependency-path: '**/packages.lock.json' for lock files in subdirectories or multiple projects.
• Restore existing dependencies with dotnet restore --locked-mode instead of adding a dependency during the cached workflow.

References:

• setup-dotnet caching requirements
• NuGet lock-file guidance
• Earlier report of the missing prerequisite

Validation:

Local checks passed against upstream commit 0226a2d78cf2be40ee3d769ec5661638461af0cc; the original article is unchanged at this branch's base:

• Content linter: node --import tsx src/content-linter/scripts/lint-content.ts --paths content/actions/tutorials/build-and-test-code/net.md.
• npm run build.
• CHANGED_FILES=content/actions/tutorials/build-and-test-code/net.md npm run test -- src/content-render/tests/render-changed-and-deleted-files.ts --run (2 tests).
• Parsed the original and corrected YAML and compared job/strategy placement with the official Actions workflow schema. Confirmed all five artifact steps and their contents are preserved.
• git diff --check.

The workflows have not been executed on a GitHub-hosted runner. The render test exited successfully with a server-shutdown warning.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.