Function validate_filepath 

pub fn validate_filepath(s: &str) -> Result<(), ValidationError>

Validate that s looks like a safe filesystem path string: non-empty, no NUL bytes, no .. parent-directory segments (path traversal). This is the structural half only — it does NOT touch the filesystem, so the caller still has to verify existence, readability and sandbox membership.

Accepted shapes:

• Relative: docs/intro.md, assets/logo.png
• Absolute (any platform): /var/uploads/x.txt, C:\Users\me\f.txt
• Trailing slash on directories

Rejected shapes:

• Empty string
• Strings containing \0 (also caught by validate_prohibit_null_characters)
• Strings with a .. segment between separators (docs/../etc/passwd, ../secret, a/../b)

Errors

ValidationError { code: "invalid_filepath", ... } on any of the rejected shapes above.