fix: wrap source_meta_config under elementary key in mock

In real dbt, node.source_meta is the full meta dict with the elementary
config nested under an "elementary" key (same structure as node.meta).
The mock was setting source_meta directly to the elementary config dict,
so _merge_elementary_from_meta couldn't find the "elementary" sub-key.

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

Author: joostboon

integration_tests/dbt_project/macros/get_anomaly_config.sql

@@ -14,7 +14,7 @@
         "config": {"elementary": model_config},
     } %}
     {% if source_meta_config is not none %}
-        {% do mock_model.update({"source_meta": source_meta_config}) %}
+        {% do mock_model.update({"source_meta": {"elementary": source_meta_config}}) %}
     {% endif %}
     {# trick elementary into thinking this is the running model #}
     {% do context.update(
@@ -36,7 +36,7 @@
         "config": {"elementary": model_config},
     } %}
     {% if source_meta_config is not none %}
-        {% do mock_model.update({"source_meta": source_meta_config}) %}
+        {% do mock_model.update({"source_meta": {"elementary": source_meta_config}}) %}
     {% endif %}
     {# trick elementary into thinking this is the running model #}
     {% do context.update(
@@ -60,7 +60,7 @@
         "config": {"elementary": model_config},
     } %}
     {% if source_meta_config is not none %}
-        {% do mock_model.update({"source_meta": source_meta_config}) %}
+        {% do mock_model.update({"source_meta": {"elementary": source_meta_config}}) %}
     {% endif %}
     {# trick elementary into thinking this is the running model #}
     {% do context.update(