test: add proposal cancellation and waitlist edge case tests

[ayinde38]

• Add four new test cases for proposal cancellation scenarios (issue C-58):

• test_cancel_proposal_by_proposer: verify proposer can cancel and second cancel returns ProposalAlreadyCancelled
• test_cancel_already_executed_proposal: ensure cancelling executed proposal returns ProposalAlreadyExecuted
• test_cancel_proposal_by_non_proposer: confirm non-proposer cancellation returns Unauthorized
• test_vote_on_cancelled_proposal: validate voting on cancelled proposal returns ProposalAlreadyCancelled

• Add helper function register_event_with_supply for waitlist tests with finite ticket supply
• Import TicketTier type in test_waitlist module for event registration with tier configuration
• Expand waitlist test coverage with edge cases for supply constraints and tier limits

Closes: #686
Closes: #687

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[Yunusabdul38]

LGTM
Thanks for your contribution