Add thinking controls to Nano Banana 2 nodes

[TotoTheDog0]

Summary

Adds configurable Gemini thinking support to the Nano Banana 2 AIO and Multi-Turn Chat nodes.

Changes

• Added thinking_level input with minimal and high options
• Added include_thoughts boolean input
• Sends types.ThinkingConfig in Nano Banana 2 requests
• Separates final response parts from thought text/images
• Adds optional thought_images output
• Updates default Nano Banana 2 model to gemini-3.1-flash-image
• Keeps gemini-3.1-flash-image-preview selectable for compatibility
• Removes the obsolete API-vs-Vertex text override in the Nano Banana 2 AIO node
• Updates README and changelog documentation

Validation

• Ran uv run python -m py_compile nodes\nano_banana_2_aio.py nodes\nano_banana_2_multiturn_chat.py
• Ran git diff --check
• Tested modified nodes (using Google Generative AI API) in ComfyUI