feat: add BytePlus and Volcengine Ark providers

[fanziqingbyte]

Summary

• add separate first-class providers for BytePlus Ark pay-as-you-go and Volcengine Ark Agent Plan
• keep credentials, regional base URLs, branding, setup links, and curated model catalogs isolated
• route both providers through the Responses API with custom endpoints
• omit OpenAI-specific reasoning.summary only for BytePlus while preserving encrypted reasoning content
• verify credentials through minimal Responses calls and document the supported providers

Provider and region matrix

Provider	Region	Base URL	Curated models	Live validation
BytePlus Ark	ap-southeast-1	https://ark.ap-southeast.bytepluses.com/api/v3	dola-seed-evolving-latest-version, dola-seed-2-1-turbo-260628	desktop smoke test passed
Volcengine Ark Agent Plan	cn-beijing	https://ark.cn-beijing.volces.com/api/plan/v3	doubao-seed-evolving, doubao-seed-2.1-turbo	confirmed after replacing an invalid local key

The two paths use different provider profiles and environment variables; neither can fall back to the other's credential.

Compatibility

• no CLI flags, public wire schemas, or existing provider defaults are changed
• stock OpenAI continues to send reasoning.summary
• Volcengine Agent Plan keeps the existing Responses request shape
• BytePlus omits the unsupported reasoning object and keeps include: ["reasoning.encrypted_content"]

Validation

• .venv/bin/pytest -q: 1185 passed, 1 skipped
• targeted provider tests: 83 passed
• npm test: 113 passed
• npm run build: passed
• packaging/build_dmg.sh: passed; unsigned arm64 DMG generated
• real desktop → sidecar → BytePlus Responses request returned OK
• Playwright: 167 passed; the pre-existing usage-chip timing case timed out in the full 9-worker run and passed when rerun in isolation