concurrency: resign election on campaign failure

[davidreis97]

Fixes #21128.

I chose the approach of calling e.Resign() immediately since it felt like a "fail safe" way to approach it without leaving dead keys. As mentioned in #21128, keeping the e.leaderSession and allowing callers to invoke e.Resign() would also work but this ensures that we deal with the potential loose keys.

I would add an integration test for this but I couldn't figure out a way to cause a failure other than context cancellation, which was already correctly calling e.Resign(). Any tips are welcome :D

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