fix: Update GitHub Copilot GPT-5.6 Terra and Luna pricing

[github-actions]

Summary

Updates GitHub Copilot catalog pricing for GPT-5.6 Luna and GPT-5.6 Terra to the post–July 30 reduced rates. The github-copilot entries still had the old pre-cut prices; OpenAI/Azure were already updated separately. Sol is unchanged.

Changes

providers/github-copilot/models/gpt-5.6-luna.toml

Field	Old	New
cost.input	1.00	0.20
cost.cache_read	0.10	0.02
cost.output	6.00	1.20
cost.cache_write	1.25	removed
tier >200K input / cache_read / output	2.00 / 0.20 / 9.00	0.40 / 0.04 / 1.80
tier cache_write	2.5	removed

providers/github-copilot/models/gpt-5.6-terra.toml

Field	Old	New
cost.input	2.50	2.00
cost.cache_read	0.25	0.20
cost.output	15.00	12.00
cost.cache_write	3.125	removed
tier >272K input / cache_read / output	5.00 / 0.50 / 22.50	4.00 / 0.40 / 18.00
tier cache_write	6.25	removed

cache_write removed because GitHub’s OpenAI pricing table only lists input / cached input / output, matching peer Copilot entries (gpt-5.4, gpt-5.5).

Evidence

• GitHub Copilot models and pricing — Luna default ≤200K: $0.20 / $0.02 / $1.20; long >200K: $0.40 / $0.04 / $1.80. Terra default ≤272K: $2.00 / $0.20 / $12.00; long >272K: $4.00 / $0.40 / $18.00. Sol unchanged at $5 / $0.50 / $30 (and $10 / $1 / $45 long). No cache-write column for OpenAI models.
• Existing first-party OpenAI entries (providers/openai/models/gpt-5.6-luna.toml, gpt-5.6-terra.toml) already use the same base rates (commented as the 2026-07-30 cut).

Validation

• Cross-checked both models against the live GitHub Copilot pricing docs page.
• Confirmed Sol pricing on that page matches the existing catalog entry (no edit).
• Did not run bun validate (shell disabled in this workflow).

Review notes

• Luna long-context threshold stays 200K per GitHub docs (OpenAI first-party Luna uses 272K; host-specific).
• Only github-copilot Luna/Terra costs were changed; Sol and other providers left alone.
• cache_write removal is intentional alignment with GitHub docs and other Copilot OpenAI models; Sol still has cache_write but was out of scope.

Closes #3964

Automated by the issue fixer: https://github.com/anomalyco/models.dev/actions/runs/30764423352

[sigco3111]

Hi! Quick question about the pricing source for GPT-5.6 Terra/Luna — did you pull the numbers from GitHub Copilot's official pricing page, or from the Azure OpenAI catalog? I've noticed the two sources often differ subtly on token-tier boundaries.

I'm maintaining a Korean dev community pricing comparison doc and want to make sure I'm citing the right source. (Btw, happy to help organize a Korean i18n contribution queue for models.dev if useful — let me know!)

Thanks!

[rekram1-node]

see description, github page