test(nostr): cover bech32 case handling

Uppercase decodes to the same payload as lowercase, mixed case is
rejected, a corrupted uppercase string still fails on the checksum rather
than on the charset, and encode/decode round-trips.

app/src/test/java/com/bitchat/android/nostr/Bech32CaseTest.kt

@@ -0,0 +1,53 @@
+package com.bitchat.android.nostr
+
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertThrows
+import org.junit.Test
+
+/**
+ * BIP-173 allows a bech32 string to be all lowercase or all uppercase, and
+ * forbids mixing the two. The uppercase form is what QR encoders emit — it fits
+ * the alphanumeric mode, which is denser — so an npub scanned from a QR code
+ * can arrive uppercased.
+ */
+class Bech32CaseTest {
+
+    private val pubkey = ByteArray(32) { (it + 1).toByte() }
+
+    @Test
+    fun `uppercase decodes to the same payload as lowercase`() {
+        val npub = Bech32.encode("npub", pubkey)
+        val (lowerHrp, lowerData) = Bech32.decode(npub)
+        val (upperHrp, upperData) = Bech32.decode(npub.uppercase())
+
+        assertEquals("npub", lowerHrp)
+        assertEquals("npub", upperHrp)
+        assertArrayEquals(pubkey, lowerData)
+        assertArrayEquals(pubkey, upperData)
+    }
+
+    @Test
+    fun `mixed case is rejected`() {
+        val npub = Bech32.encode("npub", pubkey)
+        val mixed = npub.substring(0, 6).uppercase() + npub.substring(6)
+
+        assertThrows(IllegalArgumentException::class.java) { Bech32.decode(mixed) }
+    }
+
+    @Test
+    fun `a corrupted uppercase string still fails the checksum`() {
+        val npub = Bech32.encode("npub", pubkey).uppercase()
+        val flipped = npub.dropLast(1) + if (npub.last() == 'Q') 'P' else 'Q'
+
+        assertThrows(IllegalArgumentException::class.java) { Bech32.decode(flipped) }
+    }
+
+    @Test
+    fun `round trip through encode and decode is stable`() {
+        val npub = Bech32.encode("npub", pubkey)
+        val (hrp, data) = Bech32.decode(npub)
+        assertEquals("npub", hrp)
+        assertEquals(npub, Bech32.encode(hrp, data))
+    }
+}