fix: resolve always-true is-check and redundant cast warnings in ExplainedFilterTest

Declaring 'advanced' as the base Filter type keeps the copy() regression
guard as a genuine runtime assertion instead of a compile-time triviality,
which is what the compiler was warning about.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012KG9YkeFp6zyth5J364DLF

commons/src/jvmTest/kotlin/com/vitorpamplona/amethyst/commons/relayClient/subscriptions/ExplainedFilterTest.kt

@@ -108,7 +108,9 @@ class ExplainedFilterTest {
      */
     @Test
     fun `copy preserves the purpose`() {
-        val advanced = explained().copy(since = 1_785_379_272)
+        // Typed as the base Filter so the is-check below stays a runtime assertion — with the
+        // override's covariant return type inferred, the compiler would prove it true statically.
+        val advanced: Filter = explained().copy(since = 1_785_379_272)
 
         assertTrue("copy() must stay an ExplainedFilter", advanced is ExplainedFilter)
         assertEquals(SubPurpose.NOTIFICATIONS, advanced.purposeOrNull())