Evict stale connection wrappers after django_db_modify_db_settings

[hossam1244]

Problem

When django_db_modify_db_settings replaces an entry in settings.DATABASES (the typical Testcontainers pattern), the override can silently fail if anything materialized a DatabaseWrapper earlier — import-time ORM access, an app's AppConfig.ready, or Django itself. Django's ConnectionHandler then keeps serving the stale wrapper, and because its settings cached_property references the old nested dict, setup_databases() connects to the pre-override host. connections.close_all() does not evict wrappers, and connections.databases on modern Django is only a compat property.

Fixes #1317 (full root-cause analysis and workaround in the issue — this PR turns that workaround into the fixture's own behavior).

Solution

In django_db_setup, after django_db_modify_db_settings resolves and before setup_databases():

connections.close_all()
for alias in connections:
    with contextlib.suppress(AttributeError):  # alias never materialized in this thread
        del connections[alias]
connections.__dict__.pop("settings", None)  # cached_property; no public eviction API

In-place mutation of settings.DATABASES[alias][...] was never affected (the cached settings dict is the same object); the eviction matters for replacement — which is what the docs example pattern and every Testcontainers setup do.

Testing

• tests/test_modify_db_settings.py — regression test via django_pytester: the inner module materializes connections["default"] at import time, then the session fixture replaces the alias dict with a different HOST; the test asserts the connection created by django_db_setup uses the modified host. Fails on main, passes with this change (verified red/green locally).
• Full suite: 234 passed, 6 skipped (sqlite runner, Django 6.1).

Notes for reviewers

• connections.__dict__.pop("settings", None) touches where a cached_property stores its value; there is no public API for this eviction. Happy to switch to something more official if one exists / if you'd rather fix it in Django.
• Deliberately scoped to the documented extension point rather than changing fixture ordering or adding new public API.