fix: preserve unmanaged zero model ratios

[3metaJun]

⚠️ 提交说明 / PR Notice

Important

This change was AI-assisted and manually verified against the production failure captured by the deployment gate.

📑 变更描述 / Description

The atomic provision endpoint rejected a complete merged ModelRatio map whenever an unrelated legacy model had ratio 0. That blocked configuration releases even though the zero-valued model was not exposed by any managed channel.

Allow zero only for ratios outside enabled channels in offered groups. Managed, user-visible models still require a strictly positive ratio; negative, NaN, and infinite values remain rejected.

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix)
• ✨ 新功能 (New feature)
• ⚡ 性能优化 / 重构 (Refactor)
• 📑 文档更新 (Documentation)

🔆 关联任务 / Related Issue

• Production deployment gate failure for metaproxy PR refactor(OtherSetting): change UI, improve interaction QuantumNous/new-api#96; no public issue.

✅ 提交前检查项 / Checklist

• 人工确认: The description and diff were manually reviewed.
• 非重复提交: No equivalent fork PR exists.
• Bug fix 说明: This fixes a reproduced production compatibility failure.
• 变更理解: The validation boundary and impact are understood.
• 范围聚焦: Only the atomic provision validator and regression tests changed.
• 本地验证: Targeted controller and model tests pass.
• 安全合规: No credentials or sensitive data are included.

📳 运行证明 / Proof of Work

• Regression test failed before the implementation and passes afterward.
• go test ./controller -run TestValidateMetaproxyProvisionRequest -count=1 — pass.
• go test ./model -run TestApplyMetaproxyProvision -count=1 — pass.
• git diff --check — pass.
• go test ./... reaches all packages, but the repository-wide command is not green in this checkout because the committed tree lacks web/classic/dist and an unrelated existing service mixed-mode cache test expected 2 but observed 3.