Fix error messages and detect catalog_name changes in trigger

Fixed two issues with the external_catalog_modification trigger:

1. Changed error messages to match test expectations:
   - "modifying the internal catalog is currently only supported via pg_lake_iceberg tables"
   - This matches the expected error format in test_iceberg_catalog.py

2. Added validation to detect catalog_name changes during UPDATE:
   - Prevents changing a table from external catalog to internal catalog
   - Prevents changing a table from internal catalog to external catalog
   - These operations should only be done via CREATE/DROP TABLE

Fixes CI test failures in:
- test_create_in_internal_catalog
- test_create_in_external_catalog

All external write tests still pass (9/9).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sfc-gh-mslot

pg_lake_iceberg/src/iceberg/external_metadata_modification.c

@@ -114,8 +114,25 @@ external_catalog_modification(PG_FUNCTION_ARGS)
 		prevMetadataLocationIsNull ? NULL : TextDatumGetCString(prevMetadataLocationDatum);
 
 	char	   *databaseName = get_database_name(MyDatabaseId);
+	bool		isInternalCatalog = (strcmp(catalogName, databaseName) == 0);
 
-	if (strcmp(catalogName, databaseName) == 0)
+	/* For UPDATE, check if catalog_name is being changed */
+	if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
+	{
+		Datum		oldCatalogNameDatum = heap_getattr(trigdata->tg_trigtuple, 1,
+													   trigdata->tg_relation->rd_att, &isnull);
+		char	   *oldCatalogName = TextDatumGetCString(oldCatalogNameDatum);
+		bool		wasInternalCatalog = (strcmp(oldCatalogName, databaseName) == 0);
+
+		if (isInternalCatalog != wasInternalCatalog)
+		{
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("modifying the internal catalog is currently only supported via pg_lake_iceberg tables")));
+		}
+	}
+
+	if (isInternalCatalog)
 	{
 		/*
 		 * For the current database catalog, only UPDATE is supported.
@@ -132,15 +149,13 @@ external_catalog_modification(PG_FUNCTION_ARGS)
 		{
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("INSERT to the %s catalog is only supported via CREATE TABLE ... USING iceberg",
-							databaseName)));
+					 errmsg("modifying the internal catalog is currently only supported via pg_lake_iceberg tables")));
 		}
 		else if (TRIGGER_FIRED_BY_DELETE(trigdata->tg_event))
 		{
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("DELETE from the %s catalog is only supported via DROP TABLE",
-							databaseName)));
+					 errmsg("modifying the internal catalog is currently only supported via pg_lake_iceberg tables")));
 		}
 		else
 		{