test(sync): cover the GCS parameter guard

Round trip with the built parameters, out-of-range p (0, 33, 64, 200,
255) decoding to nothing, and degenerate m (0, 1) decoding to nothing.
The p case fails without the guard: p = 0 yields 105 fabricated values.

app/src/test/kotlin/com/bitchat/android/sync/GCSFilterParameterTest.kt

@@ -0,0 +1,53 @@
+package com.bitchat.android.sync
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.util.Random
+
+/**
+ * `p` and `m` arrive off the wire — a REQUEST_SYNC carries P as a uint8 — and
+ * the decoded set decides which packets a peer is told it already has. Decoding
+ * garbage from an out-of-range parameter therefore withholds real packets, so
+ * out-of-range parameters have to decode to "peer has nothing" instead.
+ */
+class GCSFilterParameterTest {
+
+    private fun ids(n: Int): List<ByteArray> {
+        val random = Random(42)
+        return List(n) {
+            val bytes = ByteArray(16)
+            random.nextBytes(bytes)
+            bytes
+        }
+    }
+
+    @Test
+    fun `a filter round-trips with the parameters it was built with`() {
+        val params = GCSFilter.buildFilter(ids(20), maxBytes = 400, targetFpr = 0.01)
+        val decoded = GCSFilter.decodeToSortedSet(params.p, params.m, params.data)
+
+        assertTrue("expected a non-empty decode", decoded.isNotEmpty())
+        assertTrue("values must stay in range", decoded.all { it in 1 until params.m })
+        assertEquals(decoded.toList(), decoded.sorted())
+    }
+
+    @Test
+    fun `an out-of-range p decodes to nothing rather than to garbage`() {
+        val params = GCSFilter.buildFilter(ids(20), maxBytes = 400, targetFpr = 0.01)
+
+        // 64 and above wrap Kotlin's shift operators; 255 is what the byte allows.
+        for (p in listOf(0, 33, 64, 200, 255)) {
+            val decoded = GCSFilter.decodeToSortedSet(p, params.m, params.data)
+            assertTrue("p=$p should decode to nothing, got ${decoded.size} values", decoded.isEmpty())
+        }
+    }
+
+    @Test
+    fun `a degenerate m decodes to nothing`() {
+        val params = GCSFilter.buildFilter(ids(20), maxBytes = 400, targetFpr = 0.01)
+
+        assertTrue(GCSFilter.decodeToSortedSet(params.p, 0L, params.data).isEmpty())
+        assertTrue(GCSFilter.decodeToSortedSet(params.p, 1L, params.data).isEmpty())
+    }
+}