refactor(mail): early-return guard in ImapConnectionCache (#357 review)

Restructure isConnectionDrop as leading guard clauses (definite-drop
types, then a not-MessagingException early return) instead of a when
expression, per maintainer review feedback on PR #368. Behavior is
unchanged; verified by the existing ImapConnectionCacheTest suite
(all 8 cases still pass), including the FolderClosedException /
StoreClosedException cases that depend on the check running before
the MessagingException .cause guard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/org/libremail/mail/ImapConnectionCache.kt

@@ -207,10 +207,14 @@ internal class ImapConnectionCache(
      * while idle, detected on the next op's first command before any mutation is issued — is safe. A
      * copy-then-expunge move interrupted between its two halves is the rare exception left to that review.
      */
-    private fun isConnectionDrop(error: Throwable): Boolean = when (error) {
-        is FolderClosedException, is StoreClosedException, is IOException, is ConnectionException -> true
-        is MessagingException -> error.cause is IOException || error.cause is ConnectionException
-        else -> false
+    private fun isConnectionDrop(error: Throwable): Boolean {
+        // FolderClosedException/StoreClosedException are themselves MessagingException subtypes, so these
+        // definite-drop checks must run before the MessagingException guard below — otherwise they'd fall
+        // into it and get gated on a `.cause` they don't carry, instead of the unconditional `true` below.
+        if (error is FolderClosedException || error is StoreClosedException) return true
+        if (error is IOException || error is ConnectionException) return true
+        if (error !is MessagingException) return false
+        return error.cause is IOException || error.cause is ConnectionException
     }
 
     private companion object {