docs(nano-banana-edit): update pricing for nano-banana-2 migration (FDE-372)

[colin99d]

Summary

The nano-banana-edit public endpoint is being migrated to google/nano-banana-2/edit in runpod-workers/public-ep#78, because Google retires the first-generation model (gemini-2.5-flash-image) on 2026-10-02. Pricing changes from a flat $0.038 to per-resolution, so the docs need to follow.

Linear: FDE-372

⚠️ Merge this only when the endpoint itself ships. Until public-ep#78 is deployed, the live endpoint still bills the old flat $0.038.

New pricing

Resolution	Price per image
0.5K	$0.05625
1K (default)	$0.0875
2K	$0.13125
4K	$0.175

These are verified against live calls to google/nano-banana-2/edit — each resolution was invoked and the billed amount matched exactly.

Changes

public-endpoints/models/nano-banana-edit.mdx

• Pricing row and the ## Cost calculation section now show the per-resolution table
• Document the new resolution input (default 1k)
• Remove enable_safety_checker: it is not a nano-banana-2 API parameter (it exists only as a playground toggle), so it is gone from the parameter list and from all three code samples
• output.cost is derived from the output resolution, not from megapixels
• Response example shows a real price (0.0875) instead of 0.028311

public-endpoints/reference.mdx

• Reference table row: $0.038/image → $0.05625–$0.175/image, matching how the Nano Banana Pro and Nano Banana 2 rows already express resolution ranges

Out of scope / please confirm

1. Image limit corrected from 4 to 14. The page said "up to 4 images", but the endpoint has always accepted 14 (max_length: 14). Unrelated to this migration — happy to drop this bit if you would rather keep the diff strictly about pricing.
2. nano-banana-2-edit.mdx rounds 2K to $0.13 while the endpoint actually bills $0.13125. Not touched here since it is a different model page, but it is the same kind of drift.
3. aspect_ratio, output_format, enable_sync_mode and enable_base64_output are still undocumented on this page, though the endpoint accepts all four (the Nano Banana 2 page documents most of them). Left out to keep this PR about pricing; can follow up.
4. All 17 image model pages document the output field as output.image_url, but the handler's output_schema names it result. Either there is a rename layer at the public-endpoint proxy or the docs are systematically off. Worth verifying separately — not changed here.

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[colin99d]

Superseded by #843. The plan changed: instead of migrating nano-banana-edit to google/nano-banana-2/edit, the endpoint is being retired on September 28, 2026, and users are directed to the existing Nano Banana 2 Edit endpoint. Pricing on this page therefore stays at $0.038.