fix: address Copilot review round 19 — empty-string const in the web required gate

hasMissingRequiredFields treated every empty string as missing, so a required
field the schema pins to `""` — rendered read-only, and seeded with the only
value it accepts — left Execute and Open App permanently disabled. It now
matches the TUI check: an empty string counts as supplied when the property's
own `const` pins it there, and an ordinary blank required string still does
not.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/utils/jsonUtils.ts

@@ -292,7 +292,13 @@ function hasMissingIn(
       const fieldSchema = properties[field];
       return fieldSchema === undefined ? true : !admitsNull(fieldSchema);
     }
-    return value === undefined || value === "";
+    if (value === "") {
+      // A schema may pin a field to the empty string, and the form renders such
+      // a field read-only — so reporting the seeded value as missing would
+      // disable submit on a value the user cannot change (#2123).
+      return properties[field]?.const !== "";
+    }
+    return value === undefined;
   });
 }