fix: update the error message for invalid variables (#26235)

Author: Juanadelacuesta

.changelog/26235.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ui: Fixed the error message presented for invalid Variables definitions
+```

ui/app/adapters/variable.js

@@ -146,7 +146,7 @@ export default class VariableAdapter extends ApplicationAdapter {
       return new InvalidError([
         {
           detail:
-            'Invalid name. Name must contain only alphanumeric or "-", "_", "~", or "/" characters, and be fewer than 128 characters in length.',
+            'Invalid definition. Name must contain only alphanumeric or "-", "_", "~", or "/" characters, and be fewer than 128 characters in length and value should be be under 64KiB',
           status: 400,
         },
       ]);