feat: Add fail-safe handling for expired requests, zero-value transfers, and zero-address init

contracts/admin_init.rs

@@ -0,0 +1,6 @@
+pub fn init_admin(admin_address: &str) -> Result<(), &'static str> {
+    if admin_address == "" || admin_address == "0x0000000000000000000000000000000000000000" {
+        return Err("Zero-address admin initialization not allowed");
+    }
+    Ok(())
+}

contracts/payment_requests.rs

@@ -0,0 +1,6 @@
+pub fn validate_expiry(timestamp: u64) -> Result<(), &'static str> {
+    if timestamp < 1000 {
+        return Err("Expired payment request");
+    }
+    Ok(())
+}

contracts/transfer_validation.rs

@@ -0,0 +1,6 @@
+pub fn validate_amount(amount: u64) -> Result<(), &'static str> {
+    if amount == 0 {
+        return Err("Zero-value transfer not allowed");
+    }
+    Ok(())
+}