test: cover nickname truncation across surrogate and ZWJ boundaries

Pins the ASCII behaviour so the limit is provably unchanged, then covers the
cases that used to break: a split surrogate pair, a ZWJ sequence, and a name
that is one long grapheme. A sweep over four names at every limit from 1 to 20
asserts the result is always a prefix, always inside the limit, and never ends
in a lone surrogate.

app/src/test/java/com/bitchat/android/ui/ChatUIConstantsTest.kt

@@ -0,0 +1,79 @@
+package com.bitchat.android.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ChatUIConstantsTest {
+
+    @Test
+    fun `a name inside the limit is returned unchanged`() {
+        assertEquals("alice", truncateNickname("alice", maxLen = 15))
+        assertEquals("exactlyfifteen!", truncateNickname("exactlyfifteen!", maxLen = 15))
+    }
+
+    @Test
+    fun `an ascii name is still cut at the limit`() {
+        assertEquals("hello world thi", truncateNickname("hello world this is long", maxLen = 15))
+    }
+
+    /**
+     * The limit counts UTF-16 code units, so the 15th unit of this name is the
+     * high surrogate of the emoji. Cutting there leaves a lone surrogate that
+     * renders as a tofu box.
+     */
+    @Test
+    fun `a surrogate pair is never split`() {
+        val truncated = truncateNickname("abcdefghijklmn😀", maxLen = 15)
+
+        assertEquals("abcdefghijklmn", truncated)
+        assertTrue(
+            "truncated name must not end in a lone surrogate",
+            truncated.none { Character.isSurrogate(it) }
+        )
+    }
+
+    @Test
+    fun `an emoji that fits is kept whole`() {
+        assertEquals("abcdefghijkl😀m", truncateNickname("abcdefghijkl😀mno", maxLen = 15))
+    }
+
+    /** A ZWJ sequence is one grapheme, so it is kept or dropped as a whole. */
+    @Test
+    fun `a zwj sequence is not split into its parts`() {
+        val family = "👨‍👩‍👧"
+        val truncated = truncateNickname(family + " family chat", maxLen = 15)
+
+        assertEquals(family + " family", truncated)
+        assertTrue("the family must survive whole", truncated.startsWith(family))
+    }
+
+    @Test
+    fun `a name made only of one long grapheme truncates to empty rather than half a character`() {
+        val family = "👨‍👩‍👧‍👦"
+        val truncated = truncateNickname(family, maxLen = 5)
+
+        assertEquals("", truncated)
+    }
+
+    @Test
+    fun `every truncation stays within the limit and holds no partial character`() {
+        val samples = listOf(
+            "abcdefghijklmn😀",
+            "🇮🇳 india mesh",
+            "👍🏽 thumbs up all round",
+            "plain long nickname here"
+        )
+        for (name in samples) {
+            for (limit in 1..20) {
+                val truncated = truncateNickname(name, maxLen = limit)
+                assertTrue("$name at $limit exceeded the limit", truncated.length <= limit)
+                assertTrue("$name at $limit is not a prefix of the name", name.startsWith(truncated))
+                assertTrue(
+                    "$name at $limit ended in a lone surrogate",
+                    truncated.isEmpty() || !Character.isHighSurrogate(truncated.last())
+                )
+            }
+        }
+    }
+}