[FEATURE]: Add ByteDance (Volcengine Ark) Coding Plan provider

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Please add a provider for ByteDance / Volcengine Ark Coding Plan (火山方舟 Coding Plan), like the existing kimi-coding-plan, alibaba-coding-plan-cn, zhipuai-coding-plan, minimax-cn-coding-plan and tencent-coding-plan providers.

Only the pay-as-you-go volcengine provider exists today, but the Coding Plan subscription uses a different OpenAI-compatible endpoint, so subscribers cannot use it out of the box:

• Base URL: https://ark.cn-beijing.volces.com/api/coding/v3
• Auth: Ark API key (Bearer), from https://console.volcengine.com/ark
• Models: coding-plan eligible models such as doubao-seed-code

Per CONTRIBUTING.md, provider data lives in models.dev, so I can open the corresponding PR there if this is wanted.

中文:希望内置火山方舟 Coding Plan provider,其端点与按量计费的 volcengine 不同,目前需手动在 opencode.json 配置,希望能像其他家 Coding Plan 一样开箱即用。