don't ignore templated _raw_params that k=v parser failed to parse (#79913)

fixes #79862

changelogs/fragments/79862-fix-varargs.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- TaskExecutor - don't ignore templated _raw_params that k=v parser failed to parse (https://github.com/ansible/ansible/issues/79862)

lib/ansible/executor/task_executor.py

@@ -515,6 +515,10 @@ class TaskExecutor:
                                     "(see https://docs.ansible.com/ansible/devel/reference_appendices/faq.html#argsplat-unsafe)")
                 variable_params.update(self._task.args)
                 self._task.args = variable_params
+            else:
+                # if we didn't get a dict, it means there's garbage remaining after k=v parsing, just give up
+                # see https://github.com/ansible/ansible/issues/79862
+                raise AnsibleError(f"invalid or malformed argument: '{variable_params}'")
 
         # update no_log to task value, now that we have it templated
         no_log = self._task.no_log

test/integration/targets/tasks/playbook.yml

@@ -9,4 +9,16 @@
     # ensure we properly test for an action name, not a task name when cheking for a meta task
     - name: "meta"
       debug:
-        msg: Hello
+        msg: Hello
+
+    - name: ensure malformed raw_params on arbitrary actions are not ignored
+      debug:
+        garbage {{"with a template"}}
+      ignore_errors: true
+      register: bad_templated_raw_param
+
+    - assert:
+        that:
+        - bad_templated_raw_param is failed
+        - |
+          "invalid or malformed argument: 'garbage with a template'" in bad_templated_raw_param.msg