fix(opencode): preserve response model metadata

[KarmCraft]

Issue for this PR

Closes #42420

Related to #26091, but intentionally narrower: this keeps the AI SDK's structured model ID instead of arbitrary response headers.

Type of change

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

What does this PR do?

OpenCode currently discards response.modelId when adapting AI SDK stream events. This carries that value on the corresponding step-finish part and exposes it as optional modelID in the session schema and generated SDK type. Response headers are not retained.

How did you verify your code works?

• 47 focused session tests
• typechecks for packages/llm, packages/schema, and packages/opencode
• SDK regeneration
• full pre-push Turbo typecheck: 30 packages
• formatting and git diff --check

Screenshots / recordings

Not applicable; this is session metadata only.

Checklist

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

[github-actions]

Hey! Your PR title Preserve response model metadata on assistant steps 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. 👍

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
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If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.

[KarmCraft]

This contribution is still actively maintained and remains the prerequisite for pingdotgg/t3code#10726. I rebased it onto the current dev branch and force-pushed the exact-lease update at 3def0580e.

Current validation passes: 47 focused tests, the repository pre-push typecheck across all 30 participating packages, the four directly affected package typechecks, targeted oxlint with 0 errors, Prettier, and git diff --check.

The closure was the automated age/reaction cleanup rather than a code or review failure. Could a maintainer please reopen the PR for review?

[KarmCraft]

Replacement PR: #49317. The implementation was rebased onto current dev and republished because this PR could not be reopened by the contributor account.

[KarmCraft]

The current replacement for this contribution is #49571. It uses the same validated implementation head and supersedes automation-closed #49317.