Add Vercel AI Gateway provider

[rafael81]

/claim #573

Summary

• Adds a Vercel AI Gateway server provider backed by the OpenAI-compatible https://ai-gateway.vercel.sh/v1 API.
• Adds Echo SDK support via createEchoVercelAIGateway, plus exported VercelModels and model listing integration.
• Generates Vercel Gateway language model pricing from the unauthenticated /v1/models endpoint. Echo exposes these as vercel-ai-gateway/<provider>/<model> and strips the prefix before forwarding upstream so existing OpenRouter model IDs are not overwritten.
• Supports AI_GATEWAY_API_KEY and VERCEL_AI_GATEWAY_API_KEY for Gateway credentials.

Validation

• pnpm -C packages/sdk/ts run update-models:vercel
• pnpm -C packages/sdk/ts run type-check
• pnpm -C packages/sdk/ts run build
• pnpm -C packages/app/server run build
• pnpm -C packages/app/server exec vitest run src/__tests__/vercel-ai-gateway-provider.test.ts
• git diff --check

Known existing issue

• pnpm -C packages/app/server run type-check still fails on pre-existing src/utils.ts export issue: ExactEvmPayloadAuthorization is declared locally in types but not exported. After SDK build and Prisma generation, this was the only remaining server type-check error.

Disclosure

Implemented with AI assistance and manually reviewed/validated before submission.

[vercel]

@rafael81 is attempting to deploy a commit to the Merit Systems Team on Vercel.

A member of the Team first needs to authorize it.

[rafael81]

Follow-up on the red Vercel statuses: these appear to be authorization-gated deployment checks for the Merit Systems Vercel team, not code/test failures in this PR. The Vercel bot message says a team member needs to authorize the deployments first.\n\nThe local validation I ran before submission remains listed in the PR body:\n\n- pnpm -C packages/sdk/ts run update-models:vercel\n- pnpm -C packages/sdk/ts run type-check\n- pnpm -C packages/sdk/ts run build\n- pnpm -C packages/app/server run build\n- pnpm -C packages/app/server exec vitest run src/__tests__/vercel-ai-gateway-provider.test.ts\n- git diff --check\n\nThis follow-up was AI-assisted and manually reviewed.