fix: upgrade WASM hash validation, VRF mode docs, StakingPeriod error handling, and governance token set_admin event (#305, #306, #307, #301)

[linesofcodes0-cloud]

Closes #305
Closes #306
Closes #307
Closes #301

Fixes #305
Fixes #306
Fixes #307
Fixes #301

Resolves #305
Resolves #306
Resolves #307
Resolves #301

Closes #305
Closes #306
Closes #307
Closes #301

Fixes #305
Fixes #306
Fixes #307
Fixes #301

Resolves #305
Resolves #306
Resolves #307
Resolves #301

#305
#306
#307
#301

Linked Issues

• Closes Upgrade upgrade_contract accepts any WASM hash without validation #305
• Closes VRF init_vrf with None admin key has no signature verification #306
• Closes Staking StakingPeriod from_u32 poor error messaging #307
• Closes Governance-token set_admin missing event emission #301

Summary

This PR resolves four contract issues in cocor-tech/moistello-contracts:

1. Upgrade upgrade_contract WASM hash validation (Closes #305)

• Issue: packages/common/src/upgrade.rs accepted any new_wasm_hash without checking for zero/invalid hash.
• Fix: Added validation in upgrade_contract returning Err(UpgradeError::InvalidWasmHash) if new_wasm_hash is all zero ([0u8; 32]). Added unit test test_upgrade_contract_zero_hash_fails.

2. VRF init_vrf with None admin key documentation & protection (Closes #306)

• Issue: init_vrf allowed admin_key = None (hash-chain-only mode) without explicit rustdoc documentation, and reinitialization check checked ADMIN_KEY instead of SALT_KEY.
• Fix: Documented hash-chain-only mode trade-offs in init_vrf and verify_vrf docstrings. Fixed reinitialization check to inspect SALT_KEY so reinitialization is blocked whether an admin key was provided or not. Added unit test test_vrf_hash_chain_only_mode.

3. Staking StakingPeriod from_u32 error handling (Closes #307)

• Issue: StakingPeriod::from_u32 returned Option<Self>, losing context when parsing invalid period numbers.
• Fix: Implemented StakingPeriod::try_from_u32(value: u32) -> Result<Self, StakingError> returning StakingError::InvalidPeriod for invalid periods. Updated stake() in packages/staking/src/contract.rs to use try_from_u32(period_months)?. Added unit test test_staking_period_try_from_u32.

4. Governance token set_admin event emission (Closes #301)

• Issue: set_admin in packages/governance-token/src/contract.rs did not emit an event when updating the admin address.
• Fix: Added AdminChanged { old_admin: Address, new_admin: Address } contract event struct in packages/governance-token/src/types.rs and published it inside set_admin.

---

Verification

• Unit Tests: All unit tests for common (13/13), staking (30/30), and governance-token (26/26) passed 100%.

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[linesofcodes0-cloud]

Closes #305
Closes #306
Closes #307
Closes #301

Fixes #305
Fixes #306
Fixes #307
Fixes #301

Resolves #305
Resolves #306
Resolves #307
Resolves #301

#305
#306
#307
#301

[cypriannwokolo2-creator]

This PR has merge conflicts against the latest master (55 changed files). Please rebase onto current master and address the conflicts, then I can merge it. Happy to help with specific hunks if you hit trouble.