fix(events): emit PendingManagerCancelled event when replacing pending proposal

[rushikeshgarad2024-dev]

Summary of Changes

Closes #137
Reference: docs/threat-model.md v1.0, Spoof.7 (planned hardening).

Problem

Previously, calling propose_manager when a PendingManager was already active would overwrite the storage record without emitting PendingManagerCancelled. An observer watching contract events would see two consecutive ManagerProposed events without an explicit cancellation for the superseded proposal, leading to timeline ambiguity.

Solution

• Updated propose_manager in contracts/events/src/event_ops.rs to check if a pending manager is already set for event_id via storage::get_pending_manager(env, event_id).is_some().
• If an existing proposal is present, emit evt::PendingManagerCancelled { event_id }.publish(env) immediately prior to writing the replacement PendingManager and publishing ManagerProposed.
• Added unit test propose_manager_emits_cancellation_when_replacing_pending_proposal to contracts/events/src/tests/cross_contract.rs to verify replacement semantics and successful subsequent acceptance.