Fix list service of TreeView mishandled in block for FileExplorer

[Vigilans]

This PR fixes #87804.

Though getMultiSelectedResources will be deprecated as described in #86268, this PR may add some cases for not breaking current behaviors when it gets removed.

There's a further scenario where getMultiSelectedResources will return a wrong resource:

1. An extension adds a command to ExplorerItem's right click context.
2. The command handler invokes revealFileInOS, but providing a uri different from selected file
3. getMultiSelectedResources will still return selected file's uri, not the uri desired by the extension.

But this scenario seems so rare, so not included in this PR, just for a testcase when removing getMultiSelectedResources.

[isidorn]

Thank you very much for this PR. It looks good to me, merging it in.
Sorry for breaking your extension! I was not aware that extension trees depend on the reveal in OS command.