fix: replace `from_utf8_unchecked` with `from_utf8` in SQLite column name handling (#4221)

* fix: replace from_utf8_unchecked with from_utf8 in SQLite statement handle

Replace all uses of `from_utf8_unchecked` with safe `from_utf8` in the
SQLite statement handle to fix a soundness issue.

SQLite allows non-UTF-8 column names via its C API, but
`from_utf8_unchecked` assumes valid UTF-8 without checking. This can
produce invalid `&str` values through a safe public API, which is
undefined behavior in Rust.

Using `from_utf8().expect()` instead converts potential UB into a
defined panic with a clear message. There is no behavioral change for
valid UTF-8 inputs, which covers all practical usage.

Fixes launchbadge/sqlx#4192

* style: run rustfmt on handle.rs