refactor(mdb): use safe conn.prepare() for infer_schema

[lewiszlw]

Summary

Replace the manual unsafe ODBC statement lifecycle in MdbConnection::infer_schema with odbc-api's safe Connection::prepare() API.

Before

let pre = conn.preallocate()?;
let mut stmt = pre.into_handle();
let sql_text = SqlText::new(&sql);
stmt.prepare(&sql_text)?;
unsafe { stmt.execute() }?;
let cursor = unsafe { CursorImpl::new(stmt) };
let remote_schema = Arc::new(build_remote_schema(cursor)?);

After

let mut prepared = conn.prepare(&sql)?;
let remote_schema = Arc::new(build_remote_schema(&mut prepared)?);

Prepared implements ResultSetMetadata and AsStatementRef, so column metadata is available directly after prepare — no execute or cursor needed.