fix(chat): stop ArrowUp from eating an unsent multi-line prompt (#5875)

static/app.js carried a near-verbatim copy of the prompt-recall logic in
static/js/composerArrowUpRecall.js, wired as a second capture-phase
keydown listener on the same #message textarea. The copy omitted the
draft guard the module has: it called preventDefault() and
stopImmediatePropagation() unconditionally, then recalled history[0]
over whatever the user had typed.

Because it stopped immediate propagation, the copy won regardless of
registration order — if it ran first the module never saw the event, and
if it ran second the module had already declined to stop propagation on
an unmatched draft. The guard at composerArrowUpRecall.js:109 was
unreachable on the real page, so ArrowUp on a multi-line draft replaced
it with the last sent prompt instead of moving the caret up a line.

Delete the duplicate. The module keeps ownership of ArrowUp/ArrowDown
recall, which is the behavior MODULE_SUMMARY.md documents ("on an empty
composer") and the behavior tests/test_composer_arrow_up_recall_js.py
already pins via test_non_empty_composer_does_not_recall and
test_multiline_caret_navigation_preserved.

Also correct a stale comment in the module that described the deleted
behavior and contradicted the guard 35 lines above it, and add a
regression test asserting app.js does not reintroduce a second handler.

Fixes #5862

static/js/composerArrowUpRecall.js

@@ -143,9 +143,9 @@ export function wireArrowUpRecall(composer, getUserMessages, options = {}) {
       return;
     }
 
-    // ArrowUp owns prompt history in the chat composer. If the current text
-    // is not already a recalled prompt, start from newest instead of letting
-    // the browser move the caret inside the textarea.
+    // ArrowUp walks older prompts. An unmatched draft already returned above,
+    // so reaching here means the composer is empty or holds a recalled prompt
+    // — the caret-navigation case is never hijacked.
     const nextIndex = currentIndex >= 0 ? Math.min(currentIndex + 1, history.length - 1) : 0;
     const recalled = history[nextIndex];
     if (!recalled) {