fix(validation): refuse a specs argument that is not a list

tests/inputs/test_inputs.yml

@@ -49,6 +49,24 @@
         fail_msg: "[inputs no-match] got {{ inputs_nomatch_result.msg | default('') }}"
         success_msg: "[inputs no-match] OK"
 
+    - name: Refuse a spec list given as a string
+      block:
+        - name: Run the validator with specs as a string
+          validate_inputs:
+            specs: "{{ playbook_dir }}/fixture_spec.yml"
+          register: inputs_string_result
+      rescue:
+        - name: Keep the string-specs failure
+          ansible.builtin.set_fact:
+            inputs_string_result: "{{ ansible_failed_result }}"
+
+    - name: Assert the string was refused
+      ansible.builtin.assert:
+        that:
+          - inputs_string_result.msg is search('specs must be a list')
+        fail_msg: "[inputs string] got {{ inputs_string_result.msg | default('') }}"
+        success_msg: "[inputs string] OK"
+
     - name: Refuse a variable declared by two specs
       block:
         - name: Run the validator with the fixture spec twice