Feat/multi 186 187 188 189

[adetomiwa21]

Summary of Changes

This PR implements protocol fixes across the smartdrop-contracts repository:

1. Vesting Wallet Event Emission (vesting transfer_admin does not emit event #189): Added event test coverage ensuring transfer_admin emits (symbol_short!("vest"), symbol_short!("adm_xfr")).
2. FarmingPool Staked User Count (No staked user count getter in FarmingPool #188): Added staked_user_count getter and automatic tracking when users enter or completely exit positions/stakes across stake, unstake, lock_assets, unlock_assets, and emergency_withdraw.
3. FarmingPool Lock Assets Extension (lock_assets does not extend unlock_ledger when adding to existing position #187): Enforced and tested unlock_ledger extension when adding stake to an existing locked position.
4. Factory WASM Hash Validation (set_pool_wasm_hash does not validate the hash is valid WASM #186): Implemented non-zero WASM hash validation in set_pool_wasm_hash and initialize with FactoryError::InvalidWasmHash.

Closes #186
Closes #187
Closes #188
Closes #189

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