Feat/opencode zen provider

[snail-vs]

Summary

• Added OpenCode Zen as a model-aware provider.
• Routes Zen models across Responses, Anthropic Messages, and Chat Completions.
• Added Zen-specific missing-API-key messaging.
• Fixed Zen Claude authentication to use x-api-key.
• Kept OpenCode Go out of this PR.

Testing

• [ x ] cargo fmt --all -- --check
• [ x ] cargo clippy --workspace --all-targets --all-features
• cargo test --workspace --all-features
  Full workspace tests were run serially. One unrelated TUI test failed in the full suite but passed when rerun in isolation:
  non_recoverable_engine_error_enters_offline_mode

Checklist

• [ x ] Updated docs or comments as needed
• [ x ] Added or updated tests where relevant
• [ x ] Verified TUI behavior manually if UI changes
• [ x ] Harvested/co-authored credit uses a GitHub numeric noreply address

[github-actions]

Thanks @snail-vs for taking the time to contribute.

This repository is observing a maintainer-managed PR intake gate in dry-run mode, so this pull request is staying open. This note helps maintainers prepare the allowlist before any enforcement is considered.

Please read CONTRIBUTING.md for the expected contribution shape. A maintainer can grant recurring PR access by commenting /lgtm on a pull request.

[Hmbown]

Intake note: this covers the OpenCode Zen half of #1481, which is milestone-assigned to v0.9.3 (the OpenCode Go half shipped in #4420; Zen + the Messages-protocol route follow in the v0.9.3 window). Setting the milestone accordingly — no action needed from you; we'll do the full review pass when v0.9.3 opens. Thanks for taking this on!

[Hmbown]

Thanks — this is what a complete provider addition looks like: WirePolicy::ModelAware with per-model wire selection across Responses, Anthropic Messages, and Chat Completions, fail-closed handling for unknown and Gemini models, the Zen x-api-key fix with request-shape tests, secrets env-alias tests, and the parity pieces (docs/PROVIDERS.md, config.example.toml, CHANGELOG, check-provider-registry.py) all in one branch.

Where it stands: per the intake note, this is the Zen half of #1481 (Go shipped in #4420) and sits on the v0.9.3 milestone; v0.9.1 is freezing and v0.9.2 is the active lane, so the full review pass comes when v0.9.3 opens. The full CI matrix has not run on this head yet — fork PRs only get the intake gate and GitGuardian until review.

To keep the branch landable in the meantime:

1. Rebase onto current main. The July 17-18 credential-help centralization (eb20e81, ca026e3, f6a882c) moved provider credential-page logic out of crates/tui/src/config.rs into CredentialHelp in crates/config/src/provider.rs; your hunk pointing Zen/Go at https://opencode.ai/zen/ needs to land on that new surface.
2. Expect one more mechanical registry shift when the TelecomJS preset (feat: add TelecomJS provider support with configuration and catalog i… #4370 / Custom provider live catalog is never refreshed in production (scoped catalog state rewrite) #4462) lands in the v0.9.2 window ahead of this — re-derive the array counts rather than merging blindly.
3. The full-suite flake you hit (non_recoverable_engine_error_enters_offline_mode) is not on the known-papercuts list; if you still have the output, filing it as an issue would be independently useful.

Related demand your work serves: #1481 and #4526.

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