fix: swev-id: django__django-15569 clear lookup cache after unregister

[casey-brooks]

Summary

• clear stale lookup cache entries when a lookup is unregistered so transforms disappear immediately
• add regression assertions exercising lookup cache invalidation for ForeignObject and functional unique constraints
• keep constraint teardown inside the temporary registration scope so removal works once the cache is purged

Fixes #505.

Reproduction Steps

1. cd tests
2. PYTHONPATH=$PWD:$PWD/.. ../.venv/bin/python runtests.py --parallel 1 custom_lookups.tests.LookupTests.test_lookups_caching
3. PYTHONPATH=$PWD:$PWD/.. ../.venv/bin/python runtests.py --parallel 1 schema.tests.SchemaTests.test_func_unique_constraint_lookups

Observed Failures

Creating test database for alias 'default'...
Testing against Django installed in '/workspace/django/django'
Found 1 test(s).
System check identified no issues (0 silenced).
F
======================================================================
FAIL: test_lookups_caching (custom_lookups.tests.LookupTests.test_lookups_caching)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/workspace/django/tests/custom_lookups/tests.py", line 327, in test_lookups_caching
    self.assertNotIn("exactly", field.get_lookups())
AssertionError: 'exactly' unexpectedly found in {'in': <class 'django.db.models.fields.related_lookups.RelatedIn'>, 'exact': <class 'django.db.models.fields.related_lookups.RelatedExact'>, 'lt': <class 'django.db.models.fields.related_lookups.RelatedLessThan'>, 'gt': <class 'django.db.models.fields.related_lookups.RelatedGreaterThan'>, 'gte': <class 'django.db.models.fields.related_lookups.RelatedGreaterThanOrEqual'>, 'lte': <class 'django.db.models.fields.related_lookups.RelatedLessThanOrEqual'>, 'isnull': <class 'django.db.models.fields.related_lookups.RelatedIsNull'>, 'exactly': <class 'custom_lookups.tests.Exactly'>}

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)
Destroying test database for alias 'default'...

Creating test database for alias 'default'...
Testing against Django installed in '/workspace/django/django'
Found 1 test(s).
System check identified no issues (0 silenced).
F
======================================================================
FAIL: test_func_unique_constraint_lookups (schema.tests.SchemaTests.test_func_unique_constraint_lookups)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/workspace/django/django/test/testcases.py", line 1571, in skip_wrapper
    return test_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/django/tests/schema/tests.py", line 2775, in test_func_unique_constraint_lookups
    self.assertIsNone(name_field.get_transform("lower"))
AssertionError: <class 'django.db.models.functions.text.Lower'> is not None

----------------------------------------------------------------------
Ran 1 test in 0.003s

FAILED (failures=1)
Destroying test database for alias 'default'...

Testing

• cd tests && PYTHONPATH=$PWD:$PWD/.. ../.venv/bin/python runtests.py --parallel 1 custom_lookups.tests.LookupTests.test_lookups_caching
• cd tests && PYTHONPATH=$PWD:$PWD/.. ../.venv/bin/python runtests.py --parallel 1 schema.tests.SchemaTests.test_func_unique_constraint_lookups
• .venv/bin/flake8 django/db/models/query_utils.py
• .venv/bin/flake8 tests/custom_lookups/tests.py
• .venv/bin/flake8 --extend-ignore=E721 tests/schema/tests.py

[casey-brooks]

Test & Lint Summary

• cd tests && PYTHONPATH=/workspace/django:/workspace/django/.. ../.venv/bin/python runtests.py --parallel 1 custom_lookups.tests.LookupTests.test_lookups_caching (1 test, 0 failed)
• cd tests && PYTHONPATH=/workspace/django:/workspace/django/.. ../.venv/bin/python runtests.py --parallel 1 schema.tests.SchemaTests.test_func_unique_constraint_lookups (1 test, 0 failed)
• .venv/bin/flake8 django/db/models/query_utils.py (no issues)
• .venv/bin/flake8 tests/custom_lookups/tests.py (no issues)
• .venv/bin/flake8 --extend-ignore=E721 tests/schema/tests.py (no issues; E721 ignored to accommodate existing pattern)

[noa-lucent]

Verified the lookup unregister path now clears the cached lookup maps and the new tests exercise the regression for both custom lookups and functional unique constraints. Looks good.

[casey-brooks]

Test & Lint Summary

• ../.venv/bin/python runtests.py --parallel 1 custom_lookups.tests.LookupTests.test_lookups_caching: passed (1 test)
• ../.venv/bin/python runtests.py --parallel 1 schema.tests.SchemaTests.test_func_unique_constraint_lookups: passed (1 test)
• Linting not requested (not run)