chore(java): minor deferred-followups — PendingExtends TODO + ErrorCode tagging

Two untagged throw sites + one missing TODO surfaced by the prior code review.

- MetaDataLoader.resolvePendingExtends: add a TODO Javadoc note about
  cycle detection. Single-pass resolve matches TS reference behavior
  (parity, not regression), but a cycle would land silently and only
  surface at usage time. Add a one-pass cycle check when this becomes a
  real-world hazard.

- CanonicalJsonParser.loadFromStream: the fall-through catch (Exception)
  was throwing an untagged MetaDataException. Tag it with
  ERR_MALFORMED_JSON so callers see a stable conformance code regardless
  of which parse-time failure path fired.

- MetaDataRegistry.createInstance: the "Failed to create instance" throw
  was untagged. Tag with ERR_UNKNOWN so registry-instantiation failures
  carry a discoverable code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dmealing

server/java/metadata/src/main/java/com/metaobjects/loader/MetaDataLoader.java

@@ -163,6 +163,11 @@ public void addPendingExtends(PendingExtends pending) {
      * Resolve queued cross-file/forward {@code extends} refs. Mirrors the
      * TS/C# behaviour: defer the lookup until every source has populated the
      * tree, then bind. Anything still unresolved throws ERR_UNRESOLVED_SUPER.
+     *
+     * <p><b>TODO: cycle detection.</b> Single-pass resolve matches TS reference
+     * behavior (parity, not regression), but a cycle (a → b → a) would land
+     * silently and only surface at usage time. Add a one-pass cycle check at
+     * queue drain when this becomes a real-world hazard.</p>
      */
     private void resolvePendingExtends() {
         if (pendingExtends.isEmpty()) return;

server/java/metadata/src/main/java/com/metaobjects/registry/MetaDataRegistry.java

@@ -294,8 +294,10 @@ public <T extends MetaData> T createInstance(String type, String subType, String
 
         } catch (Exception e) {
             throw new MetaDataException(
-                "Failed to create instance of type: " + typeId.toQualifiedName() + 
-                " with class: " + definition.getImplementationClass().getName(), e);
+                "Failed to create instance of type: " + typeId.toQualifiedName() +
+                " with class: " + definition.getImplementationClass().getName(),
+                null, null, e, java.util.Collections.emptyMap(),
+                com.metaobjects.ErrorCode.ERR_UNKNOWN);
         }
     }