fix(restore): leave an extension's own objects to the extension

The --empty pre-clean picked its candidates by owner, on the stated
assumption that extension members are superuser-owned and would never be
selected. That holds only when a superuser installed the extension. A role
that installs one itself owns its functions, so they were listed for a
one-by-one DROP that postgres refuses:

    cannot drop function vector_in(cstring,oid,integer)
    because extension vector requires it

Under ON_ERROR_STOP that ends the whole restore, which is how a discourse
generation - it declares the vector extension - became unreplayable.

Membership now comes from pg_depend rather than from ownership. Each branch
carries its oid and classid so one NOT EXISTS covers all seven instead of
seven separate predicates, and the schema branch gets the same guard because
an extension can own a schema too. Skipping the members is enough: the dump's
CREATE EXTENSION IF NOT EXISTS finds the surviving extension either way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/e2e/test_e2e_postgres_empty_drop_hard.py

@@ -24,6 +24,7 @@ from .helpers import (
 # is not unique") and english_stem_nostop reproduces taiga's text search
 # dictionary abort (duplicate pg_ts_dict_dictname_index).
 SCENARIO_SQL = (
+    "CREATE EXTENSION pg_trgm;"
     "CREATE SCHEMA discourse_functions;"
     "CREATE TABLE discourse_functions.helper (id int);"
     "INSERT INTO discourse_functions.helper VALUES (1);"
@@ -166,6 +167,13 @@ class TestE2EPostgresEmptyDropHard(unittest.TestCase):
         )
         self.assertEqual(self._scalar("SELECT public.f(41) + public.f();"), "42")
 
+    def test_the_extension_survived_the_preclean(self) -> None:
+        self.assertEqual(
+            self._scalar("SELECT count(*) FROM pg_extension WHERE extname='pg_trgm';"),
+            "1",
+        )
+        self.assertEqual(self._scalar("SELECT similarity('abc','abc')::int;"), "1")
+
     def test_text_search_dictionary_restored_once(self) -> None:
         self.assertEqual(
             self._scalar(