win_msg: added doc about msg limit and included an explicit check for better error handling (#31078)

lib/ansible/modules/windows/win_msg.ps1

@@ -36,6 +36,10 @@ $result = @{
     wait = $wait
 }
 
+if ($msg.Length -gt 255) {
+    Fail-Json -obj $result -message "msg length must be less than 256 characters, current length: $($msg.Length)"
+}
+
 $msg_args = @($to, "/TIME:$display_seconds")
 
 if ($wait) {

lib/ansible/modules/windows/win_msg.py

@@ -51,6 +51,7 @@ options:
   msg:
     description:
       - The text of the message to be displayed.
+      - The message must be less than 256 characters.
     default: Hello world!
 author:
 - Jon Hawkesworth (@jhawkesworth)

test/integration/targets/win_msg/tasks/main.yml

@@ -25,3 +25,9 @@
     - not msg_wait_result|failed
     - msg_wait_result|changed
     - msg_wait_result.runtime_seconds > 5
+
+- name: fail to send a message > 255 characters
+  win_msg:
+    msg: "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456"
+  register: fail_too_long
+  failed_when: "fail_too_long.msg != 'msg length must be less than 256 characters, current length: 256'"