extract() no longer duplicates NULL-containing rows in shared lookups

The lookup-table insert relied on INSERT OR IGNORE and the unique index
to dedupe against existing rows, but SQLite unique indexes treat NULLs
as distinct - extracting a second table into the same lookup table
re-inserted every NULL-containing value, growing orphan rows on each
extract. The insert now also has an IS-based NOT EXISTS guard, matching
how the foreign keys themselves are resolved.

Refs https://github.com/simonw/sqlite-utils/issues/769#issuecomment-4900034150

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/changelog.rst

@@ -19,6 +19,7 @@ Unreleased
 - Fixed a bug where a failed write statement executed with ``db.execute()`` left the driver's implicit transaction open. Every subsequent write then joined that phantom transaction, which nothing committed, so their work was silently rolled back when the connection was closed. The implicit transaction opened by a failed statement is now rolled back before the exception is raised. A failed write inside a transaction opened with ``db.begin()`` or ``db.atomic()`` leaves that transaction open and untouched, as before.
 - Fixed a bug where transaction-control statements prefixed with an empty statement - ``db.query("; COMMIT")`` - or a UTF-8 byte order mark slipped past the check that rejects them, committing the caller's open transaction before raising a confusing ``OperationalError``. The keyword scanner used by ``db.query()`` and ``db.execute()`` now skips leading ``;`` and byte order marks, matching what the ``sqlite3`` driver tolerates before the first token, so these statements are rejected with a ``ValueError`` without being executed. The same fix means ``db.execute("; BEGIN")`` no longer auto-commits the transaction it just opened.
 - Documented a limitation of ``db.query()``: a ``PRAGMA`` statement that returns no rows raises a ``ValueError`` but still takes effect, because PRAGMA statements run outside the savepoint guard used to roll back other rejected statements. Use ``db.execute()`` for row-less PRAGMA statements.
+- Fixed a bug where running ``table.extract()`` more than once against the same lookup table inserted duplicate rows for values containing ``null`` - SQLite unique indexes treat ``NULL`` values as distinct, so ``INSERT OR IGNORE`` alone could not dedupe them. Each repeat extract added another copy that nothing referenced. The insert now uses an ``IS``-based ``NOT EXISTS`` guard so ``null``-containing rows match existing lookup rows.
 - ``db.add_foreign_keys()`` no longer silently ignores requested ``ON DELETE``/``ON UPDATE`` actions when a foreign key with the same columns already exists - it raises ``AlterError`` suggesting ``table.transform()``, since the actions of an existing foreign key cannot be changed in place. Exact duplicates, including actions, are still skipped so repeated calls stay idempotent. The method also now validates that compound foreign keys have the same number of columns on both sides, instead of silently discarding the extra columns.
 - ``db.ensure_autocommit_on()`` now raises ``TransactionError`` if called while a transaction is open. Assigning ``isolation_level`` commits any pending transaction as a side effect, so entering the block silently committed the caller's open transaction and made a later ``rollback()`` a no-op.
 - ``sqlite-utils migrate --stop-before`` now exits with an error if the named migration has already been applied. Previously the name passed validation but was only checked against pending migrations, so every migration after it was silently applied - the exact outcome ``--stop-before`` exists to prevent. ``Migrations.apply(db, stop_before=...)`` raises ``ValueError`` in the same situation, before applying anything.