fix(governance): reject set_execution_delay before initialize

[KorexOnchain]

Closes #603

The bug

set_execution_delay in contracts/iln_governance/src/lib.rs had a fallback branch that silently set StorageKey::Admin to the caller if it was not yet initialized:

} else {
    env.storage().instance().set(&StorageKey::Admin, &admin);
}

If initialize() was never called (or there was a race during deployment), any caller could invoke set_execution_delay and become the governance admin — gaining veto power and proposal execution rights.

The fix

• Added GovernanceError::NotInitialized (variant 21).
• Removed the else branch entirely. set_execution_delay now uses .ok_or(GovernanceError::NotInitialized)? to fetch the stored admin, returning NotInitialized if it's unset, and Unauthorized if the caller doesn't match. initialize() remains the only code path that writes StorageKey::Admin.

Tests added

• test_set_execution_delay_before_initialize_rejected — calling on an uninitialized contract panics with NotInitialized instead of silently granting admin to the caller.
• test_set_execution_delay_wrong_admin_rejected — calling with an address that isn't the configured admin panics with Unauthorized.
• test_set_execution_delay_correct_admin_succeeds — the legitimate admin path still works correctly (regression guard against an overly broad fix).

Verification

• cargo test -p iln_governance: 100 passed, 1 failed. The 1 failure (test_vote_receipt_available_within_ttl) is pre-existing and unrelated — confirmed via git stash that it fails identically on unmodified main before any of these changes.
• cargo fmt -p iln_governance -- --check: no diff.
• cargo clippy -p iln_governance --all-targets -- -D warnings: clean.

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