Add Vercel AI Gateway as a Provider

[285729101]

Summary

Closes #573

Adds the Vercel AI Gateway as a new provider, enabling Echo users to route requests through Vercel's unified AI gateway to access models from multiple upstream providers (OpenAI, Anthropic, Google, xAI, Mistral, DeepSeek, etc.) through a single endpoint.

Changes

SDK (packages/sdk/ts)

• Added packages/sdk/ts/src/supported-models/chat/vercel.ts with model definitions and per-token pricing for 25 models across 6 upstream providers (Anthropic, DeepSeek, Google, Mistral, OpenAI, xAI). Pricing is sourced from the Vercel AI Gateway API via @ai-sdk/gateway.
• Added packages/sdk/ts/src/providers/vercel.ts with createEchoVercelAIGateway() SDK provider function using @ai-sdk/gateway.
• Added packages/sdk/ts/scripts/update-vercel-models.ts script to auto-update model definitions and pricing from the gateway API (matches the pattern used by update-openai-models.ts, update-anthropic-models.ts, etc.).
• Moved @ai-sdk/gateway from devDependencies to dependencies since it is now used at runtime.
• Added update-models:vercel npm script and included it in update-all-models.
• Updated src/index.ts, src/providers/index.ts, and src/supported-models/index.ts to export the new VercelModels, VercelModel type, createEchoVercelAIGateway, and VercelAIGatewayProvider type.

Router/Server (packages/app/server)

• Added VERCEL_AI_GATEWAY to the ProviderType enum.
• Added VercelAIGatewayProvider server-side provider that routes requests to https://ai-gateway.vercel.sh/v1. Uses the OpenAI-compatible SSE format for streaming and non-streaming responses.
• Updated ProviderFactory to map Vercel provider models to the new VERCEL_AI_GATEWAY provider type and instantiate VercelAIGatewayProvider.
• Updated AccountingService to include VercelModels in the supported models list for accurate cost tracking.
• Added VERCEL_API_KEY to env.ts for gateway authentication.

Pricing

Model pricing uses the Vercel AI Gateway pricing API (already used in packages/sdk/ts/scripts/update-models.ts). The update-vercel-models.ts script fetches live pricing and regenerates the static model definitions. Models use the @provider/model ID format (e.g., @openai/gpt-4o, @anthropic/claude-sonnet-4).

Test plan

• Verify SDK builds successfully with pnpm build in packages/sdk/ts
• Verify server builds successfully with pnpm build in packages/app/server
• Verify VercelModels are exported and accessible from the SDK
• Test createEchoVercelAIGateway() creates a valid gateway provider
• Test routing a chat completion request with a Vercel model ID (e.g., @openai/gpt-4o)
• Verify cost accounting correctly calculates token costs for Vercel models
• Run pnpm update-models:vercel to confirm the update script works with the gateway API

[vercel]

@285729101 is attempting to deploy a commit to the Merit Systems Team on Vercel.

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[285729101]

@rsproule @fmhall would love your review on this when you get a chance. The gateway integration follows the same patterns as the existing providers and uses the pricing API for accurate cost tracking.