[API Nodes] Add System Instruction support & Visual Enforcement for Banana Nodes

[bigcat88]

This PR introduces a system_instruction input for Gemini nodes.

• Text Models: Default value is empty.
• Image Models ("Banana" / Gemini-3-Pro-Image): A carefully crafted system prompt has been added to enforce image generation.

Motivation & The Problem
There is a significant behavioral discrepancy between Google AI Studio and the raw Vertex AI API.

• AI Studio implicitly wraps user prompts with system instructions that force the model into "Visual Mode."
• ComfyUI uses **Vertex AI ** that executes raw prompt. Consequently, if a user provides a prompt that looks like a script, a storyboard, or a conversational request (e.g., "I need urgent help" or "3x3 Storyboard Grid"), the model often interprets it as a text-generation task. It returns a text reply or a refusal (Safety/Assistant response) instead of an image, leading to NO_IMAGE errors.

API Node PR Checklist

Scope

• Is API Node Change

Pricing & Billing

• Need pricing update
• No pricing update

If Need pricing update:

• Metronome rate cards updated
• Auto‑billing tests updated and passing

QA

• QA done
• QA not required

Comms

• Informed Kosinkadink

[bigcat88]

+label: Core

[Kosinkadink]

merging