feat: add Astraflow provider support

[ucloudnb666]

Summary

• Add Astraflow global and China endpoints to the built-in provider catalog fallback
• Register well-known provider IDs for Astraflow (astraflow, astraflow-cn)
• Mention Astraflow in the quick-start provider options

Notes

• Uses verified runtime model IDs only (gpt-5.5, deepseek-v4-pro, qwen3.7-max, o4-mini, deepseek-ai/DeepSeek-R1-0528)
• Uses conservative model limits where required by the provider schema (context=128000, output=4096)
• release_date set to "unknown" — no fabricated metadata
• China endpoint included (api.modelverse.cn/v1)

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✅ Sandbox Verification

This PR was validated in an isolated sandbox environment before submission.

Verification result

Sandbox validation passed — provider registration, schema integration, and model catalog verified.