fix(smart-contracts): fix #399 yield reward edge cases and #391 event hub access control

[Dopezapha]

Closes #391
Closes #399

PR Description

Fixes #399 by preserving fractional yield farming rewards across both global accumulator updates and per-user claims, so integer rounding does not silently discard reward dust. The reward flow still uses O(1) per-share accounting, while invalid AMM share states are rejected before reward math runs.

Fixes #391 by centralizing Event Hub access checks and requiring registered source contracts to authorize event capture. Registered contracts can now emit through the hub via Soroban contract-invoker authorization, while unregistered or forged sources are rejected.

Changed

• [Smart Contracts] Yield Farming: Reward distribution edge cases #399 Added global and per-user reward remainder tracking in src/yield_farming/src/lib.rs.
• [Smart Contracts] Yield Farming: Reward distribution edge cases #399 Added validation for negative total shares, negative user shares, and user shares exceeding total shares.
• [Smart Contracts] Yield Farming: Reward distribution edge cases #399 Added unit tests for fractional reward dust and invalid AMM share edge cases.
• [Smart Contracts] Event Hub: Add access control modifier #391 Added require_admin and require_registered_source helpers in src/event_hub/lib.rs.
• [Smart Contracts] Event Hub: Add access control modifier #391 Required source authorization before Event Hub records and re-emits captured events.
• [Smart Contracts] Event Hub: Add access control modifier #391 Added tests for unregistered source rejection and registered contract-invoker capture.
• [Smart Contracts] Event Hub: Add access control modifier #391 Fixed the Event Hub cross-contract event topic to use a valid Soroban short symbol.

[drips-wave]

@Dopezapha Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits