Reuse an existing interactive az session on disk instead of re-prompting on every restart

[shellicar]

Summary

• An interactive az identity now reuses a still-valid signed-in session found on disk after a CLI restart, instead of always triggering a fresh sign-in prompt.
• IFileSystem gained mkdir, mkdtemp, and deleteDirectoryRecursive, and writeFile now accepts an optional mode.
• The az session cache's own filesystem work (session/extension dirs, cert writes, config-dir cleanup) now goes through IFileSystem instead of direct node:fs calls.
• The az session cache tests no longer touch real disk, and moved out of the integration test tier accordingly.