Plug request_votes storage leak

[LandynDev]

Summary

• clear_request_data only ever removed the scalar request metadata (vote count / created block / hash) — the per-(request_id, validator) entries in request_votes were never cleared, so every quorum round leaked validator_count trie entries forever.
• Replaced request_votes: Mapping<(u64, AccountId), bool> + request_vote_count: Mapping<u64, u32> with a single request_voters: Mapping<u64, Vec<AccountId>>. record_vote scans/pushes; clear_request_data drops the whole vec in one op.
• get_pending_reserve_vote_count reads vec.len(). Its selector (0x3781315a) is unchanged, so contract_client.py needs no update.
• Metadata regenerated (selectors stable; only type IDs shifted from the dropped fields).

Note: this is a storage-layout change, so it only applies cleanly to a fresh deploy. Any in-flight consensus round at upgrade time will be lost — halt first if upgrading a live instance.

Test plan

• cargo contract build --release passes (48.4K optimized wasm)
• cargo test --lib passes (no unit tests present)
• E2E suite 02 against a redeployed contract