fix: add system prompt to google gen-ai request

[isaacs]

Current versions of google's genai SDK allow passing the system prompt separate from the user messages.

Update test to accurately track that we're including this in the span.

Related to JS-1218

[linear]

JS-1218 JS Google GenAI - response model missing

Error: Fixture validation failed:
llm_call (gen_ai.models):
   gen_ai.response.model: missing
   gen_ai.request.messages: mismatch (expected: {"type":"json_array","min_length":2,"items_have":["role","content"]}, got: "You are a helpful math assistant. Your anwser only with the final result.\n\nWhat is 69 + 96?")

  "dependencies": {
    "@sentry/node": "10.24.0",
    "@google/genai": "1.29.0"
  }

[sentry]

Bug: The systemInstruction in config is incorrectly passed as an array [system] instead of a string.
_{Severity: CRITICAL | Confidence: High}

🔍 Detailed Analysis

The generateContent() call will fail at runtime because the systemInstruction parameter within the config object is incorrectly passed as an array [system] instead of a string. The Google GenAI JavaScript SDK expects systemInstruction to be a string, which will lead to either a validation error from the API or an SDK error during parameter processing. This prevents the system instruction from being properly applied, causing the API call to fail.

💡 Suggested Fix

Change systemInstruction: [ system ] to systemInstruction: system in the config object to pass the system string directly, aligning with SDK documentation.

🤖 Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: sdks/js/google-genai/cases/1-simple.js#L23

Potential issue: The `generateContent()` call will fail at runtime because the
`systemInstruction` parameter within the `config` object is incorrectly passed as an
array `[system]` instead of a string. The Google GenAI JavaScript SDK expects
`systemInstruction` to be a string, which will lead to either a validation error from
the API or an SDK error during parameter processing. This prevents the system
instruction from being properly applied, causing the API call to fail.

_{Did we get this right? 👍 / 👎 to inform future reviews.
Reference ID: 6609577}

[isaacs]

This actually can be a PartListUnion, so a Part object, string, or array of Part objects and strings. (Annoying, I know, but including it this way helped flush out a bug in the JS SDK, actually.)