fix(rust): a bad title mode reaches the form as a sentence

The router's own isIn check answered a mode it did not know with
express-validator's "Invalid value" before titles.validateSettings could
say which words are allowed. Found on the walk; the router now checks shape
only, as every other phase-17 route does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/router/admin/rust.router.js

@@ -159,8 +159,10 @@ adminRustRouter.put(
   /* #swagger.responses[404] = { description: 'No such server' } */
   requireRole('admin'),
   param('id').isString().isLength({ min: 1, max: 64 }),
-  body('mode').optional().isIn(['first', 'all', 'upto']),
+  // Shape only. The words, the bounds and the per-rule sentences are
-  body('rules').isArray({ max: 10 }).withMessage('rules is a list of at most ten'),
+  // `titles.validateSettings`'s, in the controller, so a bad mode reaches the
+  // form as a sentence rather than as "Invalid value".
+  body('rules').isArray().withMessage('rules is a list'),
   validate,
   admin.putTitles,
 )