Remaining panicking UTF-8 reads: read_stored, get_rows_by_file, function args #1630

#1593 (PR #1612) made query results error with DbError::NotUtf8 on invalid-UTF-8 TEXT instead of panicking. Three paths still use the panicking row.get::<rusqlite::types::Value> conversion:

• packages/rust/src/db.rs read_stored and get_rows_by_file (watcher path): a declared table whose DEFAULT or generated column yields invalid UTF-8 can panic.
• packages/rust/src/functions.rs argument conversion (~line 134): unverified whether rusqlite catches the panic inside the callback.

Expected: the same named error as #1593, no panic.