add_foreign_keys() errors instead of silently dropping action changes

The existing-foreign-key dedup compared only columns and other_table, so
requesting an FK that already exists with different ON DELETE/ON UPDATE
actions was a silent no-op - and with add_foreign_key() raising "already
exists", there was no signal that the requested actions were dropped.
An exact match (including actions) is still skipped for idempotency;
a mismatch now raises AlterError pointing at table.transform().

Also validates that compound foreign keys passed as 4-tuples have the
same number of columns on both sides - extra other-columns were being
silently discarded, e.g. ("t", ("a",), "other", ("a", "b")) created a
single-column key referencing just "a".

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

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

docs/python-api.rst

@@ -1636,6 +1636,8 @@ Here's an example adding two foreign keys at once:
 
 This method runs the same checks as ``.add_foreign_keys()`` and will raise ``sqlite_utils.db.AlterError`` if those checks fail.
 
+Foreign keys that already exist are silently skipped, so repeated calls are idempotent - but only if they match exactly. Requesting a foreign key that exists with different ``ON DELETE``/``ON UPDATE`` actions raises ``AlterError``: use ``table.transform()`` to change the actions of an existing foreign key.
+
 .. _python_api_index_foreign_keys:
 
 Adding indexes for all foreign keys