Fix XEP-0060 PubSub compliance: purge-node ClassCastException and missing payload size enforcement

[Copilot]

Two XEP-0060 §1.30.0 compliance bugs in PubSubEngine, both covered by new unit tests in PubSubEngineTest.

Bug 1: purgeNode throws ClassCastException on collection nodes

The isCollectionNode() guard came after an unconditional (LeafNode) node cast, so purging a collection node crashed instead of returning the spec-required error.

Fix: Swap the order — check isCollectionNode() first, then cast.

// Before (throws ClassCastException for collection nodes):
if (!((LeafNode) node).isPersistPublishedItems()) { ... }
if (node.isCollectionNode()) { ... }

// After (XEP-0060 §8.5.3.1 — correct error returned):
if (node.isCollectionNode()) { ... }                       // feature-not-implemented / purge-nodes
if (!((LeafNode) node).isPersistPublishedItems()) { ... }  // feature-not-implemented / persistent-items

Bug 2: publishItemsToNode ignores maxPayloadSize

LeafNode.maxPayloadSize was stored and exposed but never checked — a TODO comment acknowledged this gap. XEP-0060 §7.1.3.5 requires returning <not-acceptable/> + <payload-too-big/> when the payload exceeds the configured limit (default 10 MB).

Fix: Added byte-length check (UTF-8) in the item-processing loop after existing payload structure validation:

if (payload != null) {
    final int payloadSize = payload.asXML().getBytes(StandardCharsets.UTF_8).length;
    if (payloadSize > leafNode.getMaxPayloadSize()) {
        Element pubsubError = DocumentHelper.createElement(
            QName.get("payload-too-big", "http://jabber.org/protocol/pubsub#errors"));
        sendErrorPacket(iq, PacketError.Condition.not_acceptable, pubsubError);
        return;
    }
}

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[guusdk]

These changes look good to me. Can we rewrite the commit message(s) (possibly split them into two distinct commits) to include the JIRA identifiers of the tickets that I created for each of the issues fixed just now?

[akrherz]

rebased and split commits...