feat(provider): add data residency header support for OpenAI Enterprise

[DusKing1]

Issue for this PR

No existing issue — this addresses a gap in OpenAI Enterprise support for non-US regions using Codex OAuth.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds the x-openai-internal-codex-residency HTTP header to OpenAI Codex provider requests in packages/opencode/src/plugin/codex.ts.

Problem: OpenAI Enterprise workspaces with US data residency reject Codex API requests from non-US regions with 401 "Workspace is not authorized in this region". OpenCode's Codex auth plugin shares the identical OAuth flow, client ID, and endpoint as OpenAI's own Codex CLI — but is missing this one header.

Fix: Read enforce_residency from provider options and set x-openai-internal-codex-residency in the chat.headers hook. This mirrors exactly how Codex CLI handles it in codex-rs/core/src/default_client.rs. Users configure it via opencode.json:

{
  "provider": {
    "openai": {
      "options": {
        "enforce_residency": "us"
      }
    }
  }
}

The provider options schema already uses .catchall(z.any()), so no schema changes are needed.

How did you verify your code works?

• Verified the header is added to outgoing requests when enforce_residency is set in provider config
• Confirmed the header is NOT added when the option is absent (no impact on non-Enterprise users)
• Cross-referenced with Codex CLI source to ensure header name and value match exactly

Screenshots / recordings

N/A — backend-only change, no UI impact.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title feat(plugin/codex): add data residency header support for OpenAI Enterprise doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[DusKing1]

I verified this change locally and it works as expected. After configuring enforce_residency for the OpenAI provider, requests consistently include the x-openai-internal-codex-residency header.

[GreyElec]

it looks good and looking forward to see this in the official version