fix: make keyword_token safe by validating UTF-8 input

[kmr-ankitt]

This PR fixes an unsound usage of unsafe { str::from_utf8_unchecked(word) } in the public function keyword_token in mod.rs.

The function now uses std::str::from_utf8(word).ok()? to safely handle invalid UTF-8, eliminating the unsoundness.
No logic or API changes.
Code compiles and tests pass (where possible).

Closes: #1859

[penberg]

Hey @kmr-ankitt, there are other places that call from_utf8_unchecked() too, almost certainly for performance reasons. As far as I can tell, it's probably better to attempt to fix this in the upstream repository and then merge back, instead of fixing parts of it in libsql.git. Happy to also discuss fixing this in https://github.com/tursodatabase/limbo where we hard forked the parser.