🐞 Contest and Selection Can Leak Memory

### Is there an existing issue for this?

- [ ] I have searched the existing issues

### Current Behavior

If you create a PlaintextBallotSelection or PlaintextBallotContest in C# and immediately dispose of them, the underlying C++ won't get disposed and you'll have a memory leak. The underlying C++ objects currently only get disposed when you add them to a ballot and then dispose of the ballot.

### Expected Behavior

_No response_

### Steps To Reproduce

_No response_

### Environment

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### Anything else?

_No response_