fix: Issues with DBConnection class

[RisingOrange]

Our custom DBConnection class can be used as a context manager like this:
https://github.com/ankipalace/ankihub_addon/blob/b76d2118c8afee21ae6ad589846efbd5ac64e801/tests/addon/test_unit.py#L1131-L1133

(ankihub_db.connection() returns an instance of DBConnection)
One could expect that if there was an exception in the context, the operations would be rolled back. This is how it works for sqlite3.Connection when it's used as a context manager. However this is currently not the case for DBConnection. This will be fixed by this PR.

For context: The intention behind supporting the usage of DBConnection as a context manager was mainly to improve performance. Instantiating a DBConnection object requires opening a new database connection, which is relatively expensive. It also can't be re-used when not used as a context manager:
https://github.com/ankipalace/ankihub_addon/blob/b76d2118c8afee21ae6ad589846efbd5ac64e801/tests/addon/test_unit.py#L1114-L1121
So using the DBConnection as a context manager was a way to improve performance when multiple queries need to be performed in a loop.

Related issues

fix: Issues with DBConnection class

Proposed changes

• Adjust DBConnection to rollback changes when there is an exception in the context
• Add docstrings to the DBConnection class and its methods
• Add tests for the DBConnection class

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