feat: add BytePlus/Volcengine Ark provider support #63480

Feature request

Add support for the BytePlus/Volcengine Ark API as a new provider in OpenCode.

The Ark API is OpenAI-compatible, so it can extend openai-compatible with Volcengine-specific defaults (region, thinking budget, prompt cache key via X-Ark-Prompt-Id header).

Proposed approach

Split into focused PRs:

1. Ark provider (packages/ai): providers/ark.ts, export, tests
2. Catalog entries: submit BytePlus/Volcengine model data upstream to models.dev instead of bundling locally
3. Core wiring (packages/core): aisdk-native mapping, variant handling, provider icon
4. provider-error.ts: minor comment fix (optional, separate PR)

Note: the initial attempt (#63471) was too large and included a 1,700-line local catalog and a generic GET /models discovery plugin. Per maintainer feedback, the catalog should go upstream and the discovery plugin should be discussed separately.