fix(provider): route big-pickle through chat completions

[mturac]

Issue for this PR

Closes #28138

Type of change

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

What does this PR do?

Normalizes the opencode big-pickle models.dev entry so it uses the chat-completions-compatible SDK route instead of the messages-style route. That keeps requests away from /zen/v1/messages, which is the failing path reported in the issue, and adds a regression test for this normalization case.

How did you verify your code works?

• bun test test/provider/provider.test.ts --timeout 30000 from packages/opencode
• bun run --cwd packages/opencode typecheck
• bunx prettier --check packages/opencode/src/provider/provider.ts packages/opencode/test/provider/provider.test.ts
• git diff --check
• bunx oxlint packages/opencode/src/provider/provider.ts packages/opencode/test/provider/provider.test.ts (0 errors; existing warnings only)

Screenshots / recordings

N/A; provider routing change.

Checklist

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

[github-actions]

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

[toastsandwich]

lgtm