Fix SMB3 directory leasing parent tracking

[gabrielg5]

Fix SMB3 GlobalFileTable parent tracking when directory leasing is negotiated.

• Avoid referencing an uninitialized parentDir for shallow paths.
• Distinguish opening the share root from opening a root-level file.
• Include the share name in non-DFS global file keys.
• Reuse existing parent entries instead of raising an exception.
• Add regression coverage for share-root, root-level, nested, and repeated-parent cases.

This PR fixes #2256