pushdown: make FilterApplier HTTP-state-serializable

FilterApplier is held in several table-function producers' StreamState. Over
HTTP that state is serialized into a continuation token, but FilterApplier had
only a private 2-arg constructor and final fields, so the field-based CBOR
serializer couldn't reconstruct it on /exchange. Add a no-arg constructor and
drop final on filterBytes/joinKeysIpc (cached stays transient, re-decoded
lazily). No behaviour change for the in-memory transports.

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

Author: rustyconover

vgi/src/main/java/farm/query/vgi/pushdown/FilterApplier.java

@@ -18,10 +18,19 @@
  */
 public final class FilterApplier {
 
-    private final byte[] filterBytes;
-    private final List<byte[]> joinKeysIpc;
+    // Non-final + a no-arg constructor so the HTTP transport's field-based CBOR
+    // state serializer can round-trip a FilterApplier held in a producer's
+    // StreamState across /init -> /exchange. cached is transient (re-decoded
+    // lazily after a round-trip).
+    private byte[] filterBytes;
+    private List<byte[]> joinKeysIpc;
     private transient PushdownFilters cached;
 
+    /** No-arg constructor for state deserialization; {@link #from} is the API. */
+    private FilterApplier() {
+        this.joinKeysIpc = List.of();
+    }
+
     /**
      * Create an applier over the init-time pushdown payload; decoding is deferred
      * to first {@link #apply}.