fix(api-nodes-nanobana): raise error when not output image is present

[bigcat88]

When Gemini refuses a prompt (e.g. content policy) or responds with text only, the image nodes returned a silent black 1024x1024 tensor.
Now raises a ValueError with the model's text response so users understand why generation failed.

• Affects GeminiImage, GeminiImage2, and GeminiNanoBanana2 nodes
• Preserves black placeholder for optional thought_image output in NanoBanana2

Note: Text-only prompts (e.g. what is 2+2?) now raise an error instead of returning a black image.
This is intentional (we can't distinguish clear text replies from replies with error) - for text generation, use the Google Gemini LLM node instead.

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

[coderabbitai]

📝 Walkthrough

Walkthrough

The get_image_from_response function in comfy_api_nodes/nodes_gemini.py now implements improved error handling when no image tensors are collected and thought is False. Instead of returning a zero-filled tensor, the function attempts to extract the model's text output and raises a ValueError with the response included. If no text is present, it raises a generic ValueError suggesting a retry with a different response modality setting.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly describes the main change: raising an error when no output image is present, which aligns with the changeset's core modification to error handling in Gemini image nodes.
Description check	✅ Passed	The PR description clearly explains the changes: raising ValueError with model response when no image is generated, affecting specific Gemini nodes, with intentional behavior changes documented.

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