[RPP][AI] Add a condition to the Auto Labelling Agent prompt

It  sometimes returns multi-line responses, let's try to prevent it.

Bug:None
Change-Id: I2d1044c3f230cb31377441b9bb9a063aa179e5ca
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6414390
Commit-Queue: Alina Varkki <[email protected]>
Commit-Queue: Jack Franklin <[email protected]>
Auto-Submit: Alina Varkki <[email protected]>
Reviewed-by: Jack Franklin <[email protected]>

Author: AlinaVarkki

front_end/models/ai_assistance/agents/PerformanceAnnotationsAgent.ts

@@ -38,5 +38,5 @@ Generate a concise label (max 60 chars, single line) describing the *user-visibl
 - Base the description only on the information present within the call tree data.
 - Prioritize brevity.
 - Only include third-party script names if their identification is highly confident.
-- Always use "responsiveness" rather than "user interaction responsiveness".
+- Very important: Only output the 60 character label text, your response will be used in full to show to the user as an annotation in the timeline.
 `;