fix(guardrails/azure): guard undefined systemMessages in shieldPrompt

[ankit25bcs10610]

What

In the Azure shieldPrompt guardrail:

const systemMessages = context.request?.json?.messages?.filter(
  (message) => message.role === 'system'
);
...
// guarded correctly here:
if (Array.isArray(systemMessages) && systemMessages.length > 0) { ... }
...
const request = {
  userPrompt,
  ...(systemMessages.length > 0 ? { documents: textArray } : {}), // NOT guarded
};

systemMessages is undefined whenever the request has no messages array (e.g. a completion-style payload). The documents spread uses a bare systemMessages.length, throwing TypeError: Cannot read properties of undefined (reading 'length') — and this line is outside the surrounding try, so the handler rejects instead of returning a clean verdict.

Fix

Apply the same Array.isArray(systemMessages) guard already used a few lines above.

npm run format:check passes.