Add acceptance coverage for an untyped plugin secret

Restores the untyped plugin secret case in plugin.robot, which was lost
when the atests were refactored on top of it. Uses the renamed
Get Keyword Call Banner Text helper. Against the type-map-only detection
the banner shows the plugin secret in clear text, so the case guards the
name rule end to end alongside the unit test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SuiwgM8Rt9kd6K7TBtFMDz

Author: Snooz82

atest/test/09_Plugins/plugin.robot

@@ -105,4 +105,8 @@ Keyword Call Banner Covers Plugin Keywords
     ...    ValueError: Direct assignment of values or variables as 'secret' is not allowed.*
     ...    Plugin Login With Credentials    css=input#username    ${PLUGIN_SECRET}
     Get Keyword Call Banner Text    ==    Plugin Login With Credentials \ \ \ css=input#username \ \ \ ***
+    Run Keyword And Expect Error
+    ...    ValueError: Direct assignment of values or variables as 'secret' is not allowed.*
+    ...    Plugin Login Without Type Hints    css=input#username    ${PLUGIN_SECRET}
+    Get Keyword Call Banner Text    ==    Plugin Login Without Type Hints \ \ \ css=input#username \ \ \ ***
     [Teardown]    Show Keyword Banner    None