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?

• 44 focused session tests
• typechecks for packages/llm, packages/schema, and packages/opencode
• SDK regeneration
• git diff --check

The full Windows pre-push typecheck is blocked by the upstream packages/enterprise/src/custom-elements.d.ts symlink being checked out as plain text. The affected package checks pass; CI can exercise the symlink on Linux.

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. 👍