fix(storage): make VikingFS.mkdir() actually create the target directory

[ze-mu-zhou]

Description

VikingFS.mkdir() calls _ensure_parent_dirs(path) to create all parent
directories, but never calls agfs.mkdir(path) to create the target directory
itself. The method unconditionally returns without performing the actual mkdir
operation, making every mkdir() call a no-op.

This affects TreeBuilder._move_directory_in_agfs, DirectoryInitializer,
and import_ovpack. The system may appear to work only because write_file
independently calls _ensure_parent_dirs.

Related Issue

Fixes #90

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Replace the bare return at the end of mkdir() with
  await asyncio.to_thread(self.agfs.mkdir, path) to actually create the
  target directory
• Add 5 unit tests covering normal creation, exist_ok behavior, and
  parent-before-target call ordering
• Update tests/README.md with test_mkdir.py entry

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

The bug was verified by calling viking_fs.mkdir("viking://resources/test_dir")
followed by viking_fs.stat() on the same URI, which raised
AGFSClientError: no such file or directory. See issue #90 for full
reproduction steps and output.

[CLAassistant]

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