fix(app): keep the @ menu open while it searches - #59425

The @ menu in the composer no longer jumps or empties out on every keystroke.
Typing after @ shows files, agents and skills that match. While the next file search was in flight, the menu dropped its file rows, so it shrank to just the agents and skills, then grew back when results arrived. Agents, skills and the slash menu are not affected.

🔍 Origin: seen in real use, typing @re… in the desktop app. Same fix as #59424 on beta; dev has the identical code.

❌ Before

1. Type @re. File results for re show.
2. Type s. ❌ The file rows vanish at once, so the menu shrinks (or shows "No matching items").
3. About 150 ms plus the server's reply later, files for res arrive and the menu grows back.

✅ After

1. Type @re. File results for re show.
2. Type s. The re results stay in place while res is searched.
3. The res results replace them in one step. The highlighted row stays highlighted if it is still in the list.

- keystroke -> file rows = []        -> wait for search -> file rows = new results
+ keystroke -> keep last file rows   -> wait for search -> file rows = new results
+ menu closed, empty query, or a space -> file rows = []

📊 Proof

Before, typing @research with a 250 ms search:

After:

The new component test fails on the current branch (old rows gone while the next search is pending) and passes with the change, 10 of 10 repeats.

Visual

<svg xmlns="http://www.w3.org/2000/svg" width="240" height="72" viewBox="0 0 240 72" fill="none">
  <style>
    .caret { animation: blink 1s steps(1) infinite; }
    .row2 { animation: swap 2.4s ease-in-out infinite; }
    @keyframes blink { 50% { opacity: 0; } }
    @keyframes swap { 0%, 40% { opacity: 1; } 50% { opacity: .6; } 60%, 100% { opacity: 1; } }
    @media (prefers-reduced-motion: reduce) { .caret, .row2 { animation: none; } }
  </style>
  <rect x="20.5" y="4.5" width="199" height="34" rx="6" stroke="currentColor" stroke-opacity=".25"/>
  <rect x="25" y="9" width="190" height="11" rx="3" fill="currentColor" fill-opacity=".1"/>
  <rect x="30" y="13" width="70" height="3" rx="1.5" fill="currentColor" fill-opacity=".6"/>
  <g class="row2"><rect x="30" y="27" width="96" height="3" rx="1.5" fill="currentColor" fill-opacity=".55"/></g>
  <rect x="20.5" y="42.5" width="199" height="25" rx="6" stroke="currentColor" stroke-opacity=".35"/>
  <text x="30" y="59" font-family="ui-monospace, monospace" font-size="10" fill="currentColor" fill-opacity=".8">@res</text>
  <rect class="caret" x="55" y="50" width="1" height="11" fill="currentColor"/>
</svg>

👀 Read: interaction.ts, the file search. It no longer tags results with the query they came from, so the list can briefly show files for the previous query; that is the point. The rest is routine.

_{The clips use the composer story with a simulated 250 ms file search.}