propose_manager: emit a cancellation event when replacing a pending proposal

[0xdevcollins]

Source: docs/threat-model.md v1.0, Spoof.7 (planned hardening).

propose_manager (event_ops.rs:198) overwrites any existing PendingManager without emitting PendingManagerCancelled. An observer following events sees two ManagerProposed entries and no cancellation for the first, so the timeline is ambiguous.

• Emit PendingManagerCancelled{event_id} before writing the replacement (or refuse while one is pending and require cancel_pending_manager first)
• Test the event sequence

[omeruahelisha8-png]

Hi Maintainer,
I'd like to contribute by resolving this issue. I've reviewed the details and am ready to get started.
Kindly assign it to me. Thanks!

[abojeEdwin]

Hi maintainer, I've gone through the issue and I have a clear understanding of the fix for this issue. I'd love to be assigned to this issue. Thank you

[prosperceed]

Greetings, Maintainer. I see exactly what this issue is pointing at, and I am more than glad to bring you the solution. Thank you!

[guptakumarranjeet150]

Hi there,

I would like to resolve this issue. Here is my proposed execution plan:

• Implementation: Structure Soroban contract handlers for propose_manager: emit a cancellation event when replacing a pending proposal following project idioms and storage invariants.
• Testing: Add Soroban test cases checking unauthorized invocations, edge values, and state updates.

Ready to deliver a clean, tested PR within 12–24 hours. Please assign!