fix: replace from_utf8_unchecked with from_utf8 in SQLite column name handling

[barry3406]

Summary

Fixes #4192

The SQLite driver uses from_utf8_unchecked() to convert C strings returned by SQLite's C API (column names, table names, etc.) into Rust &str values. This is a soundness violation: SQLite does not guarantee that these strings are valid UTF-8, and from_utf8_unchecked on non-UTF-8 input produces an invalid &str, which is undefined behavior in Rust.

What's wrong

SQLite's C API functions like sqlite3_column_name(), sqlite3_column_table_name(), sqlite3_column_origin_name(), sqlite3_column_database_name(), sqlite3_column_decltype(), sqlite3_bind_parameter_name(), and sqlite3_sql() can all return non-UTF-8 strings. The previous code used from_utf8_unchecked on these return values, which:

1. Violates Rust's safety invariant that all &str values must be valid UTF-8
2. Can be triggered through a safe public API (no unsafe required by the caller)
3. Constitutes undefined behavior per the Rust reference

The fix

All 8 call sites in sqlx-sqlite/src/statement/handle.rs are replaced:

• from_utf8_unchecked(...) becomes from_utf8(...).expect("descriptive message")
• The unused from_utf8_unchecked import is removed

This converts potential undefined behavior into a defined panic with a clear error message identifying which SQLite C API function returned non-UTF-8 data.

Behavioral impact

• For valid UTF-8 inputs (all practical usage): Zero behavioral change. from_utf8 returns Ok and .expect() unwraps it, producing the same &str as before.
• For invalid UTF-8 inputs: Instead of silently producing an invalid &str (UB), the program panics with a clear message. This is the correct behavior since the rest of the codebase assumes valid UTF-8 strings.

[abonander]

Turning these into panics is a step up from undefined behavior but long-term we might want to make this a recoverable error.

This is fine to merge as-is, I just wanted to get that on record.