test(governance-polls): verify vote change updates tally without double counting

[Fury03]

Add a regression test covering a holder who changes their vote before a poll
expires. Wallet A votes option 0 with 10 keys, then switches to option 1.
Asserts option 0 tally decrements by 10, option 1 increments by 10, and total
vote weight stays unchanged, confirming the previous weight is removed rather
than double-counted.

Summary

• Add changing_vote_before_expiry_removes_previous_weight_without_double_counting
  to creator-keys/tests/governance_polls.rs
• Buys 10 keys for a single holder, casts a vote for option 0, then re-votes for
  option 1
• Verifies the intermediate tally (option 0 = 10, total = 10), then the final
  tally (option 0 = 0, option 1 = 10, total = 10), proving weight moves rather
  than accumulates
• Test-only change; no contract logic, storage, or event schema touched

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item
• Added or updated creator-keys unit/integration tests for every changed contract behavior, including failure paths for new or reachable ContractError variants
• Ran cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace, or explained exactly why a command was not run
• Reviewed persistent storage changes against docs/storage-key-invariants.md; any storage layout change includes a migration/backward-compatibility note — N/A, no storage changes
• Confirmed event names, topic order, payload field order, and field meanings remain compatible with docs/contract-event-conventions.md, or documented the breaking change and versioning plan — N/A, no event changes
• Updated docs for any changed public contract interface, read-only method, event schema, storage behavior, fee logic, or deployment workflow — N/A, test-only change
• Scope stays limited to one contract concern and does not include unrelated formatting, lockfile, generated artifact, or dependency changes
  Closes Add regression test for governance poll vote weight updating correctly when voter changes their vote #539