[5.0.1] Clear events when DbContext is disposed or returned to the pool #23288
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Fixes #23108
Description
We added three new .NET events on DbContext in 5.0. These events are not cleared when the DbContext is disposed, which can lead to memory leaks.
Customer Impact
Potential memory leak. (We had a similar bug in the early days of EF6 that resulted in several hard-to-find customer memory leaks.)
How found
Internal code review.
Test coverage
We were missing coverage for this. Added in this PR.
Regression?
No; these events are new in 5.0.
Risk
Very low. Set the event handlers to null when the context is disposed (and hence it is invalid to use it anymore and any operation on it will call ObjectDisposedException. Also quirked.