Add type hints to action and test plugins and to plugin utils; fix some bugs, and improve input validation (#11167)

* Add type hints to action and test plugins and to plugin utils. Also fix some bugs and add proper input validation.

* Combine lines.

Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>

* Extend changelog fragment.

* Move task_vars initialization up.

---------

Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>

plugins/test/a_module.py

@@ -38,7 +38,11 @@ _value:
   type: boolean
 """
 
+import typing as t
+from collections.abc import Callable
+
 from ansible.plugins.loader import action_loader, module_loader
+from ansible.errors import AnsibleFilterError
 
 try:
     from ansible.errors import AnsiblePluginRemovedError
@@ -46,12 +50,14 @@ except ImportError:
     AnsiblePluginRemovedError = Exception  # type: ignore
 
 
-def a_module(term):
+def a_module(term: t.Any) -> bool:
     """
     Example:
       - 'community.general.ufw' is community.general.a_module
       - 'community.general.does_not_exist' is not community.general.a_module
     """
+    if not isinstance(term, str):
+        raise AnsibleFilterError(f"Parameter must be a string, got {term!r} of type {type(term)}")
     try:
         for loader in (action_loader, module_loader):
             data = loader.find_plugin(term)
@@ -65,7 +71,7 @@ def a_module(term):
 class TestModule:
     """Ansible jinja2 tests"""
 
-    def tests(self):
+    def tests(self) -> dict[str, Callable]:
         return {
             "a_module": a_module,
         }