Fix symlink handling in CopyDirectory method

[black-desk]

Fixes #3234

This change adds proper support for symbolic links
in the CopyDirectory method.

NOTE: This patch was developed with GitHub Copilot
assistance. I has NO C# experience at all and
is not familiar with actions/runner. This change
has not been tested in a local environment. Please
review this patch carefully.

Signed-off-by: Chen Linxuan me@black-desk.cn

[Copilot]

Pull Request Overview

This PR enhances the CopyDirectory method to properly detect and preserve symbolic links when copying directories and files.

• Adds early detection for directory symlinks and creates matching links in the target.
• Updates file-copy logic to skip already up-to-date files and supports file symlinks.
• Removes the duplicate sourceDir declaration and centralizes its initialization.

Comments suppressed due to low confidence (1)

src/Runner.Sdk/Util/IOUtil.cs:392

• The new symlink-handling branches (for directories at lines 395–408 and files at lines 426–434) lack unit tests. Please add non-trivial L0 tests under Test/L0 to verify both directory and file symlink scenarios.

            // Get the file contents of the directory to copy.

[Copilot]

[nitpick] For file symlinks, add a check to skip recreating an existing link when the target matches, similar to the directory case, to avoid unnecessary filesystem operations.

[black-desk]

I think this logic may have been covered by the section above that checks for the existence of target

[black-desk]

I don't even have a full C# development environment, so if there's anything the project maintainers would like to change, it's perfectly fine to push it directly to my branch and merge it, and I hope this will be fixed soon.