Reject odd-length and 0x-prefixed hex the way iOS does

dataFromHexString used length/2, so an odd-length string silently
dropped the trailing nibble. Match iOS Data(hexString:): trim, strip an
optional 0x/0X prefix, reject odd length, and treat empty as empty bytes.

app/src/test/java/com/bitchat/android/util/HexStringTest.kt

@@ -0,0 +1,58 @@
+package com.bitchat.android.util
+
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class HexStringTest {
+
+    @Test
+    fun `decodes even-length lowercase hex`() {
+        assertArrayEquals(
+            byteArrayOf(0x0a, 0xbc.toByte()),
+            "0abc".dataFromHexString()
+        )
+    }
+
+    @Test
+    fun `rejects odd-length hex instead of truncating`() {
+        // Previously length/2 silently dropped the trailing nibble.
+        assertNull("abc".dataFromHexString())
+        assertNull("0ab".dataFromHexString())
+    }
+
+    @Test
+    fun `strips optional 0x prefix like iOS`() {
+        assertArrayEquals(
+            byteArrayOf(0xde.toByte(), 0xad.toByte()),
+            "0xdead".dataFromHexString()
+        )
+        assertArrayEquals(
+            byteArrayOf(0xbe.toByte(), 0xef.toByte()),
+            "0Xbeef".dataFromHexString()
+        )
+    }
+
+    @Test
+    fun `empty string and empty after prefix yield empty array`() {
+        assertArrayEquals(ByteArray(0), "".dataFromHexString())
+        assertArrayEquals(ByteArray(0), "0x".dataFromHexString())
+        assertTrue("  ".dataFromHexString()!!.isEmpty())
+    }
+
+    @Test
+    fun `rejects non-hex characters`() {
+        assertNull("zz".dataFromHexString())
+        assertNull("0xgg".dataFromHexString())
+    }
+
+    @Test
+    fun `round-trips with hexEncodedString`() {
+        val original = byteArrayOf(0x00, 0x7f, 0xff.toByte())
+        val encoded = original.hexEncodedString()
+        assertEquals("007fff", encoded)
+        assertArrayEquals(original, encoded.dataFromHexString())
+    }
+}