fix(FHE): guard getDecryptResult and getDecryptResultSafe against uninitialized (zero) handles

[amathxbt]

Summary

Impl.getDecryptResult and Impl.getDecryptResultSafe in contracts/FHE.sol accept any bytes32 handle without checking whether it is initialized (non-zero). Passing a zero handle forwards uint256(0) to the task manager, which may coincidentally match a real pending or completed decrypt task at slot 0, returning a wrong plaintext the caller treats as valid.

Bug

// Before
function getDecryptResult(bytes32 input) internal view returns (uint256) {
    return ITaskManager(TASK_MANAGER_ADDRESS).getDecryptResult(uint256(input));
    // ↑ input == 0 → queries task slot 0 in the task manager
    //   If slot 0 holds a real completed task, caller receives its result silently.
}

function getDecryptResultSafe(bytes32 input) internal view returns (uint256 result, bool decrypted) {
    return ITaskManager(TASK_MANAGER_ADDRESS).getDecryptResultSafe(uint256(input));
    // ↑ same issue — may return (value, true) for slot 0
}

Impact: Contracts that accidentally call these functions with an uninitialized euintX field get a non-zero, apparently-valid result instead of an error. This can silently corrupt application state (e.g., a balance read from an uninitialized storage slot returns a non-zero plaintext).

Fix

• getDecryptResult: revert with a clear message when the handle is zero.
• getDecryptResultSafe: return (0, false) (semantically "not yet decrypted") for zero handles, consistent with the function's safe semantics.

function getDecryptResult(bytes32 input) internal view returns (uint256) {
    require(Common.isInitialized(input), "FHE: getDecryptResult called with uninitialized handle");
    ...
}

function getDecryptResultSafe(bytes32 input) internal view returns (uint256 result, bool decrypted) {
    if (!Common.isInitialized(input)) return (0, false);
    ...
}