docs: add NestM AI SDK monitoring guide

[kauandotnet]

Summary

• document registry-wide provider monitoring through @nestm/ai-sdk
• show OpenAI and Anthropic module composition, AiSdkService calls, dashboard routes, and warnings
• clarify registry bypasses, Gateway mapping, availability policy, and language-model-only coverage
• make the NestJS 12 prerelease install command copy-paste complete

Verification

• pnpm run verify
• isolated TypeScript 7 consumer compile against the published packages
• Nest application-context bootstrap and ChatService resolution