Proofreader API: Update API shape for multiple labels per correction.

This is part of the preparation for the model switching from Nano + LoRA
to TinyGemma. WPT test updated.

Bug: 452978387
Change-Id: I4984482b3c766e1ce9c25f9a50c7b31f7553be86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7548599
Reviewed-by: Nathan Memmott <memmott@chromium.org>
Reviewed-by: Mike Wasserman <msw@chromium.org>
Commit-Queue: Queenie Zhang <queeniezhang@google.com>
Cr-Commit-Position: refs/heads/main@{#1600725}

Author: QueenieZqq

ai/proofreader/proofreader-proofread.tentative.https.window.js

@@ -44,7 +44,8 @@ promise_test(async (t) => {
   assert_equals(typeof result, 'object');
   assert_not_equals(result.correctedInput, input);
   assert_greater_than(result.corrections.length, 0);
-  assert_not_equals(result.corrections[0].type, undefined);
+  assert_not_equals(result.corrections[0].types, undefined);
+  assert_greater_than(result.corrections[0].types.length, 0);
 }, 'Proofreader.proofread() returns correction types when requested');
 
 promise_test(async (t) => {