add multiregion pricing for gpt 5.6 family

[prabhash-varma]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

• New model pricing
• Update existing pricing
• New model configuration
• Bug fix

Source Verification

Source Link: [Add link here]
https://developers.openai.com/api/docs/pricing

Checklist

• I have validated the JSON using jq or an online validator
• I have verified that prices are in cents per token (not dollars)
• I have included the source link above
• I have signed the CLA (if first-time contributor)

Related Issue

Fixes # (issue)

[Copilot]

Pull request overview

Adds multi-region pricing entries for the OpenAI gpt-5.6-* model family in the pricing catalog, enabling region-specific overrides while preserving existing default pricing.

Changes:

• Added custom_pricing.regions["us,eu,au,ca,jp,in,sg,kr,gb,ae"] blocks for gpt-5.6-sol, gpt-5.6-terra, and gpt-5.6-luna.
• Updated region-specific token prices across standard, batch, flex, and priority execution modes with tiered pricing (lte-272k, gt-272k where applicable).

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