feat: add Astraflow (UCloud) provider presets

[ucloudnb666]

Summary

Adds two new LLM provider presets for Astraflow by UCloud (优刻得) — an OpenAI-compatible AI model aggregation platform that supports 200+ models.

What changed

llmprovider/presets.yml — two new ProviderPreset entries:

Key	Locale	Endpoint
astraflow	(global)	https://api-us-ca.umodelverse.ai/v1
astraflow_cn	zh-cn	https://api.modelverse.cn/v1

Both use type: openai because Astraflow is fully OpenAI-API-compatible — no new SDK, no new Go package, no new connector code needed.

Default models pre-populated

Each preset ships with 6 representative models enabled/disabled appropriately:

• DeepSeek V3 (enabled) — fast general-purpose
• DeepSeek R1 (enabled) — reasoning
• GPT-4o, GPT-4o Mini, Claude Sonnet 4.6, Gemini 2.0 Flash (disabled by default)

Sign-up / API key

• Global: https://astraflow.ucloud-global.com (env: ASTRAFLOW_API_KEY)
• China: https://astraflow.ucloud.cn (env: ASTRAFLOW_CN_API_KEY)

No breaking changes

• Existing presets and providers are untouched.
• The locale: zh-cn field on astraflow_cn ensures it is only surfaced to Chinese-locale users via GetPresetsForLocale, matching the pattern used by kimi_cn, qwen_cn, etc.
• Zero runtime impact — presets are UI-only templates; no connector is registered until a user explicitly creates a provider from the preset.