fix: raise ExportFailedError in _get_target_index for multi-container TaskDef

Mirror the build path: when len(ContainerDefinitions) > 1 and
Metadata.ContainerName is unset, sam package now raises ExportFailedError
instead of silently packaging the first container. Prevents the
wrong-container overwrite in standalone sam package invocations.

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

Author: singledigit

samcli/lib/package/packageable_resources.py

@@ -719,10 +719,14 @@ def _get_target_index(self, container_defs):
                 ),
             )
         if len(container_defs) > 1:
-            LOG.warning(
-                "TaskDefinition has multiple containers but Metadata.ContainerName is not set; "
-                "packaging the first container. Add 'ContainerName: <name>' to the resource Metadata "
-                "to avoid ambiguity."
+            raise exceptions.ExportFailedError(
+                resource_id="",
+                property_name=self.PROPERTY_NAME,
+                property_value="",
+                ex=ValueError(
+                    f"TaskDefinition has {len(container_defs)} containers but Metadata.ContainerName is not set. "
+                    f"Add 'ContainerName: <name>' to the resource Metadata to specify which container to package."
+                ),
             )
         return 0