[openai] Update GPT-5.6 Terra and Luna pricing

[Nauxie]

Summary

• update GPT-5.6 Terra pricing after OpenAI’s 20% reduction
• update GPT-5.6 Luna pricing after OpenAI’s 80% reduction
• apply the new rates to aliases and dated snapshots across standard, long-context, Batch, Flex, Fast/priority, and regional pricing

Sources

• https://openai.com/index/advancing-the-price-performance-frontier-with-gpt-5-6/
• https://developers.openai.com/api/docs/pricing

Validation

• npx eslint . --ext .json --quiet
• npx prettier --check pricing/openai.json
• validated every file in pricing/ and general/ with jq empty
• verified the semantic diff only changes the four intended model entries and preserves additional-unit pricing

[Copilot]

Pull request overview

Updates the OpenAI pricing catalog for GPT-5.6 Terra and GPT-5.6 Luna to reflect the newly announced price reductions, including their dated snapshot entries, while keeping additional-unit pricing unchanged.

Changes:

• Reduced GPT-5.6 Terra token and cache token prices by 20% across base pricing_config and all custom_pricing region/execution-mode tiers.
• Reduced GPT-5.6 Luna token and cache token prices by 80% across base pricing_config and all custom_pricing region/execution-mode tiers.
• Applied the same updates to the dated snapshots gpt-5.6-terra-2026-07-09 and gpt-5.6-luna-2026-07-09.

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