fix: harden hex string parsing (#646)

Improve Data(hexString:) to handle edge cases:
- Reject odd-length strings (return nil)
- Support optional 0x/0X prefix
- Trim leading/trailing whitespace
- Handle empty strings correctly

Add comprehensive unit tests for hex parsing.

bitchat/Protocols/BinaryEncodingUtils.swift

@@ -23,20 +23,42 @@ extension Data {
         return digest.map { String(format: "%02x", $0) }.joined()
     }
     
+    /// Initialize Data from a hex string.
+    /// - Parameter hexString: A hex string, optionally prefixed with "0x" or "0X".
+    ///   Whitespace is trimmed. Must have even length after prefix removal.
+    /// - Returns: nil if the string has odd length or contains invalid hex characters.
     init?(hexString: String) {
-        let len = hexString.count / 2
+        var hex = hexString.trimmingCharacters(in: .whitespaces)
+
+        // Remove optional 0x prefix
+        if hex.hasPrefix("0x") || hex.hasPrefix("0X") {
+            hex = String(hex.dropFirst(2))
+        }
+
+        // Reject odd-length strings
+        guard hex.count % 2 == 0 else {
+            return nil
+        }
+
+        // Reject empty strings
+        guard !hex.isEmpty else {
+            self = Data()
+            return
+        }
+
+        let len = hex.count / 2
         var data = Data(capacity: len)
-        var index = hexString.startIndex
-        
+        var index = hex.startIndex
+
         for _ in 0..<len {
-            let nextIndex = hexString.index(index, offsetBy: 2)
-            guard let byte = UInt8(String(hexString[index..<nextIndex]), radix: 16) else {
+            let nextIndex = hex.index(index, offsetBy: 2)
+            guard let byte = UInt8(String(hex[index..<nextIndex]), radix: 16) else {
                 return nil
             }
             data.append(byte)
             index = nextIndex
         }
-        
+
         self = data
     }
 }