Show a warning on a cache access when a `CachedSession` context manager has been exited

Closes https://github.com/requests-cache/aiohttp-client-cache/issues/241

MRE

import asyncio
from aiohttp_client_cache import CachedSession, SQLiteBackend

cache = SQLiteBackend(cache_name="./test_cache.sqlite")

async def task(n: int):
    url = f"https://httpbin.org/ip?count={n}"
    async with CachedSession(cache=cache) as session:
        return await session.get(url, ssl=False)

async def tasks(arr: list):
    tasks = [task(i) for i in arr]
    return await asyncio.gather(*tasks)

asyncio.run(tasks(list(range(10))))

1st run (there is no cache file yet)

We can see the warning inside get_connection() after the yield:

❯ python .gather_sessions.py
/usr/lib64/python3.13/contextlib.py:221: UserWarning: Cache access after closing the `Cachedsession` context manager is discouraged and can be forbidden in the future to prevent errors related to a closed database connection.
  await anext(self.gen)
Traceback (most recent call last):
[...]
sqlite3.ProgrammingError: Cannot operate on a closed database.

2nd run (a cache file already exists)

We can see the warning inside the def read():

/usr/lib64/python3.13/contextlib.py:221: UserWarning: Cache access after closing the `Cachedsession` context manager is discouraged and can be forbidden in the future to prevent errors related to a closed database connection.
  await anext(self.gen)
Traceback (most recent call last):
[...]
sqlite3.ProgrammingError: Cannot operate on a closed database.

Notes

Not related to this PR and the mentioned issue, but Redis and MongoDB must have the def close() method too because the connections must be closed.

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.17%. Comparing base (26bf273) to head (fb00280). Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
aiohttp_client_cache/backends/sqlite.py	50.00%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #290 +/- ## ========================================== - Coverage 96.51% 96.17% -0.35% ========================================== Files 10 10 Lines 1063 1072 +9 Branches 186 188 +2 ========================================== + Hits 1026 1031 +5 - Misses 28 30 +2 - Partials 9 11 +2 ```

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requests-cache / aiohttp-client-cache