fix: Preserve embedded NUL characters in TEXT values

[LiGuolongHAN]

Fix handling of SQLite TEXT values containing embedded NUL (\0) characters.

Previously, some JS ↔ SQLite string conversions treated TEXT as NUL-terminated, which could truncate values such as "A\0漢😄".

This change uses explicit UTF-8 byte lengths when passing strings into SQLite, and SQLite-reported byte lengths when reading them back.

It also adds regression tests for:

prepared statement parameters and query results;
JavaScript SQL function arguments and return values.

The new regression tests fail on the current master and pass with this fix. CI is green.