feat: add Vercel AI Gateway provider

[clawdvader]

/claim #573

Summary

• add a TypeScript SDK createEchoVercelAIGateway provider using @ai-sdk/gateway
• support AI Gateway /language-model requests in the Echo router by reading the gateway model/stream headers
• add a VercelAIGatewayProvider that forwards to https://ai-gateway.vercel.sh/v1/ai with AI_GATEWAY_API_KEY or VERCEL_OIDC_TOKEN
• add Vercel AI Gateway model/pricing entries to SDK model listings and Echo accounting

Verification

• pnpm --filter @merit-systems/echo-typescript-sdk build
• pnpm --filter @merit-systems/echo-typescript-sdk type-check
• pnpm --filter echo-server build
• pnpm --filter echo-server format:check

Note: pnpm --filter echo-server type-check still reports an existing unrelated export error in src/utils.ts (ExactEvmPayloadAuthorization from types) after Prisma generation, but the server build succeeds.

[vercel]

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