Add comprehensive tests for error scenarios in `TestVoteExtension`.

[tzdybal]

Add comprehensive tests for error scenarios in TestVoteExtension.

The current implementation of TestVoteExtension tests the happy path where everything functions as expected. It would be beneficial to include tests for error scenarios, such as what happens when the ExtendVote or PrepareProposal methods fail. Would you like me to help by adding these tests or should I open a GitHub issue to track this task?

Originally posted by @coderabbitai[bot] in https://github.com/rollkit/rollkit/pull/1641#discussion_r1591492635