Check and drop if stale connection when saving task result in db

[lode-braced]

No description provided.

[Copilot]

Pull request overview

This PR adds protection against stale database connections when storing Celery task results. It calls Django's close_if_unusable_or_obsolete() method before saving results to ensure the connection is still valid, particularly for long-running tasks that may exceed the configured connection max age.

Key Changes:

• Added connection validation logic in _store_result() to close stale connections before saving
• Added test coverage to verify stale connections are properly detected and replaced
• Configured CONN_MAX_AGE: None for test databases to enable persistent connection testing

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
django_celery_results/backends/database.py	Added close_if_unusable_or_obsolete() call and imported connections from django.db
t/unit/backends/test_database.py	Added test to validate stale connection handling, imported time and connections
t/proj/settings.py	Configured CONN_MAX_AGE: None for all test database connections

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[lode-braced]

@auvipy the copilot description beat me to the punch: I noticed some of our very long running celery tasks getting this exception - the work they do is not on the db connection of the task thread, and so that connection remains idle until the end of the task, at which point the server already kicked it off for being idle for too long. Main consequence is that I had to modify the tests to run outside of the usual pytest-django transaction, since the connection dropping does not play nice with being inside a transaction.