fix(postgres): restore sanitizeDocuments call removed in #5536 (#5566)
PR #5536 accidentally removed the sanitizeDocuments call when rewriting
addVectors for batch upsert support. This reintroduced a bug originally
fixed in PR #3367 where NULL characters (0x00) in document content cause
Postgres insert errors.
This restores the sanitization step to remove NULL characters before
inserting documents into the database.