vfs: fix AddVirtual ignoring isDir - #9949

What does this change do?

VFS.AddVirtual takes an isDir argument, but it always called Dir.AddVirtual with false. Adding a virtual directory therefore added a file, and nothing could then be added inside it. This passes isDir through, as pointed out in #9310.

The only caller today is the VFS cache, which always passes false, so mount and serve behave as before. The fix matters for any caller that adds directories.

Testing

• TestVFSAddVirtual adds a virtual file, then a virtual directory, then a file inside that directory, and checks each one with Stat. On master the directory comes back as a file, and adding the file inside it fails with "file already exists".
• I ran it on local, which goes through the StatParent branch, and with -remote :memory:, which goes through the mkdirAll branch used by backends that can't have empty directories.
• These pass: go build ./..., make quicktest, go test ./vfs/... (local and :memory:), go vet ./vfs/ and golangci-lint run ./vfs/....

Linked issue

#9310. This fixes the isDir bug confirmed there. It does not add the rc command the issue asks for.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate. (No doc change needed: internal function, and its comment already says "file or dir".)
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend. n/a, not a backend change.
• This Pull Request is ready for review.