fix(property-token): enforce bridge multisig expiry and operator quorum hygiene

[PatrickKish1]

Summary

• Always set expires_at from timeout_blocks or BridgeConfig.default_timeout_blocks, and enforce expiry on both sign_bridge_request and execute_bridge via a shared helper that marks the request expired, clears the pending slot, unlocks a zero-address lock, and emits BridgeRequestExpired.
• Count only current operators toward quorum (removed operators’ signatures no longer count); refuse remove_bridge_operator when it would drop below min_signatures_required.
• Unit tests for expired sign/execute (+ unlock), duplicate signature rejection, and mid-request operator rotation; ink contract build fixed (no_main, event topic limit on BridgeRequestCreated).

Closes #610

Test plan

• cd meridian-contracts/contracts/property-token && cargo +nightly test --lib
• cargo +nightly contract build
• Confirm expired initiate → sign returns RequestExpired and token can be re-bridged
• Confirm quorum lock → advance past expiry → execute returns RequestExpired and token unlocked
• Confirm same operator signing twice → AlreadySigned
• Confirm remove operator mid-request drops their vote; add + sign with new operator to complete