Fix SELECT on tables with bytea columns hanging in process worker

[AlexZeitler]

Summary

• psycopg2 returns memoryview objects for bytea columns which cannot be pickled through multiprocessing.Pipe
• The worker's send() silently swallowed the serialization error, leaving the client waiting forever (spinner)
• Convert memoryview to bytes in CursorBasedAdapter.execute_query() before rows are returned, so they can be safely pickled
• Added unit tests for the sanitization logic

Test plan

• Unit tests for _sanitize_cell and _sanitize_row pass
• Connect to PostgreSQL 17
• Create table with bytea column and insert rows (see setup SQL in SELECT on table with bytea column hangs when executed as single statement #161)
• Run SELECT * FROM bytea_repro; as single statement — should return 3 rows immediately
• Run SELECT id, name FROM bytea_repro; — should still work
• Run both statements together — should still work

Fixes #161