vfs: add robust split_parent_and_name and refactor path parsing/creation

[AMDRIP]

Motivation

• Consolidate repeated path-splitting logic into a single robust helper that trims trailing slashes and correctly handles root and edge cases.
• Fix O_CREAT handling in vfs_open to create files in the resolved parent directory rather than trying to create with the full path on the root vnode.
• Reduce duplicated code and improve error handling (release vnodes on failure) across multiple VFS operations.

Description

• Added a new split_parent_and_name helper that returns bool and properly trims trailing slashes, preserves root semantics, and splits a path into dir_out and name_out.
• Updated vfs_open to resolve the parent directory via vfs_resolve_path and call parent->ops->create(parent, filename, ...) when O_CREAT is set, and to release the parent vnode on all paths; also release the opened vnode if open fails.
• Replaced manual inline path parsing in vfs_stat, vfs_unlink, vfs_write_file, vfs_mkdir, and vfs_rename with calls to the new split_parent_and_name and added validation for its return value.
• Removed the old duplicated split_parent_and_name implementation and updated callers to handle failure cases consistently.

Testing

• Built the project with make to validate compilation, which completed successfully.
• Executed the VFS unit test suite via make test-vfs, which passed.
• Ran filesystem integration tests (make test-fs) to exercise create, write, stat, readdir, unlink, mkdir, and rename flows, which also passed.

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Codex Task