[AI] Ensure the Accessibility Agent correctly understands n/a report results

Previously, if some report result was n/a, the agent assumed that the report passed.
Let's make sure that it knows that n/a indicates mossing data or a failure.

Result: https://screenshot.googleplex.com/3gUHWBvEiysUh7E

Bug: 498942928
Change-Id: I9c755e4b324b744036b781ea7a0f51d7ad62cb9a
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7734960
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
Auto-Submit: Alina Varkki <alinavarkki@chromium.org>
Commit-Queue: Alina Varkki <alinavarkki@chromium.org>

Author: AlinaVarkki

front_end/models/ai_assistance/agents/AccessibilityAgent.ts

@@ -62,6 +62,7 @@ Your role is to help users understand and fix accessibility issues found in Ligh
 # Constraints
 * **CRITICAL**: ALWAYS call a tool before providing an answer if an element path is available.
 * **CRITICAL**: You are an accessibility agent. NEVER provide answers to questions of unrelated topics such as legal advice, financial advice, personal opinions, medical advice, or any other non web-development topics.
+* **CRITICAL**: If the Lighthouse report shows scores as "n/a" or indicates a failure, it means the data is missing or the run failed. Do NOT assume that the page passed or has no issues.
 
 ## Response Structure
 
@@ -462,7 +463,13 @@ export class AccessibilityAgent extends AiAgent<LHModel.ReporterTypes.ReportJSON
   #getInitialPayload(context: ConversationContext<LHModel.ReporterTypes.ReportJSON>): string {
     const report = context.getItem();
     const formatter = new LighthouseFormatter();
-    return `# Lighthouse Report:\n${formatter.summary(report)}\n${formatter.audits(report, 'accessibility')}\n`;
+    const summary = formatter.summary(report);
+    const audits = formatter.audits(report, 'accessibility');
+    const allFailed = Object.values(report.categories).every(category => category.score === null);
+    if (allFailed) {
+      return '**CRITICAL**: The Lighthouse report failed to record or all category scores are error/unavailable (n/a). This indicates a failed run or missing data.';
+    }
+    return `# Lighthouse Report:\n${summary}\n${audits}`;
   }
 
   override async enhanceQuery(query: string, lhr: ConversationContext<LHModel.ReporterTypes.ReportJSON>|null):