feat: add Vercel AI Gateway provider

[mojoxr]

Closes #573
/claim $1000

Summary:

• Adds Vercel AI Gateway provider support to the server
• Adds Vercel provider wrappers for TS, React, and Next SDKs
• Adds Vercel model catalog generation and tiered pricing support
• Routes AI SDK Gateway requests while stripping Echo's local vercel/ prefix upstream

Validation:

• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm --filter @merit-systems/echo-typescript-sdk run type-check
• corepack pnpm --filter @merit-systems/echo-react-sdk run type-check
• corepack pnpm --filter @merit-systems/echo-next-sdk run type-check
• corepack pnpm --filter echo-server run type-check

[vercel]

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

A member of the Team first needs to authorize it.