Fix iceberg detection in clone when source schema is not cached

load_cached_relation returns None during cross-schema clone operations
because the source schema is not in the adapter cache. Fall back to
querying Snowflake directly via SHOW TABLES to detect iceberg format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jecolvin

dbt-snowflake/src/dbt/include/snowflake/macros/materializations/clone.sql

@@ -3,8 +3,7 @@
 {% endmacro %}
 
 {% macro snowflake__create_or_replace_clone(this_relation, defer_relation) %}
-    {%- set source_relation = load_cached_relation(defer_relation) -%}
-    {%- set is_iceberg = source_relation is not none and source_relation.is_iceberg_format -%}
+    {%- set is_iceberg = snowflake__is_iceberg_table(defer_relation) -%}
     create or replace
       {% if is_iceberg -%}
         iceberg
@@ -15,3 +14,20 @@
       clone {{ defer_relation }}
       {{ "copy grants" if config.get("copy_grants", false) }}
 {% endmacro %}
+
+{% macro snowflake__is_iceberg_table(relation) %}
+    {%- set source_relation = load_cached_relation(relation) -%}
+    {%- if source_relation is not none -%}
+        {{ return(source_relation.is_iceberg_format) }}
+    {%- endif -%}
+
+    {#- Cache miss: query Snowflake directly -#}
+    {%- set show_sql -%}
+        show tables like '{{ relation.identifier }}' in {{ relation.database }}.{{ relation.schema }}
+    {%- endset -%}
+    {%- set results = run_query(show_sql) -%}
+    {%- if results and results | length > 0 -%}
+        {{ return(results.columns.get('is_iceberg').values()[0] in ('Y', 'YES')) }}
+    {%- endif -%}
+    {{ return(false) }}
+{% endmacro %}