GitHub Copilot 5.6 models (gpt-5.6-luna/sol/terra) return 403 Forbidden

[zhy-1976]

Bug

When using GitHub Copilot as the provider in opencode, the newer 5.6 model family returns 403 Forbidden from the chat completions endpoint:

• github-copilot/gpt-5.6-luna
• github-copilot/gpt-5.6-sol
• github-copilot/gpt-5.6-terra

Other models on the same account/token (4.1, 5, 5.5, claude-*, etc.) work correctly.

Reproduction

1. opencode auth login → GitHub Copilot (any account with 5.6 access, e.g. Copilot Enterprise)
2. opencode run --model github-copilot/gpt-5.6-luna 'hello'
3. Request fails with HTTP 403.

Root cause

When no Copilot-Integration-Id request header is supplied, GitHub's Copilot API infers the integration id from the OAuth client id. opencode's OAuth CLIENT_ID (Ov23li8tweQw6odWQebz) is registered internally as copilot-language-server, which is not authorized for the 5.6 model family.

Verified via curl with the same token:

Copilot-Integration-Id header	5.6 model result
(omitted)	403
copilot-language-server	403
vscode-chat	200 OK

Suggested fix

Explicitly send Copilot-Integration-Id: vscode-chat on Copilot requests, with an env override (e.g. OPENCODE_COPILOT_INTEGRATION_ID) for advanced users.

PR: #36574

[rekram1-node]

No this breaks our agreement with copilot team you CANNOT change the integration id that is incorrect and wrong.