feat(llm): add OpenCode Zen polish provider, default deepseek-v4-flash

[zhou-zhichao]

Goal

OpenCode is available as a Less Computer CLI backend, but is missing from Settings → AI providers for dictation polish. Add OpenCode Zen as a polish LLM provider with deepseek-v4-flash over the OpenAI-compatible /zen/v1/chat/completions endpoint.

Usage

1. Settings → AI providers → add channel → OpenCode Zen
2. Paste an OpenCode API key from https://opencode.ai/auth
3. Keep the model as deepseek-v4-flash

For an OpenCode Go subscription, change the same card's Base URL to https://opencode.ai/zen/go/v1 and use the Go console key.

The thinking toggle uses DeepSeek's thinking.type (off by default). A custom endpoint whose host contains opencode.ai gets the same fallback.

Implementation

• Register OpenCode in the shared Core cloud provider list and provider descriptors, including its default endpoint, model, and credential policy.
• Keep thinking controls and their tests in crates/openless-core/src/polish.rs.
• Add the frontend label and translations; channel defaults come from Core descriptors.

Verification

Run from openless-all/app:

• cargo test --locked -p openless-core — 886 passed, 1 ignored.
• npm run build — TypeScript and Vite build passed.
• node scripts/frontend-test-runner.mjs — all 69 test files passed.

Regression coverage includes provider registration and defaults, API key requirements at the default endpoint, thinking enabled/disabled for the preset and a custom Go endpoint, and frontend consumption of Core defaults.

Scope

Does not change Less Computer / OpenCode CLI, and does not change ASR.

[H-Chris233]

开始审查

[H-Chris233]

暂缓合并本 PR，优先完成 #1039。OpenCode Zen 的模型分别使用 Chat Completions、Responses 和 Messages，而当前实现固定走 Chat Completions。待 #1039 支持按渠道手动选择请求格式后，再更新本 PR 并验证各协议路径。