Add validation for VeraCrypt volume inputs

[srichs]

Motivation

• Improve robustness and error messages when creating or mounting volumes by validating user-provided inputs before invoking the VeraCrypt CLI.
• Prevent common errors such as non-positive sizes and missing parent directories for created volumes, and invalid mount points on non-Windows systems.

Description

• Add @staticmethod _validate_size to ensure size is a positive integer and raise ValueError otherwise.
• Add @staticmethod _validate_volume_parent_dir to ensure the parent directory of volume_path exists and raise VeraCryptError if it does not.
• Validate non-Windows mount_point paths in mount_volume by calling _check_path when a mount point is provided.
• Invoke _validate_size and _validate_volume_parent_dir in create_volume before building and running the create command.

Testing

• No automated tests were run for this change.

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Codex Task