test: verify MiniMax models through Context7Agent - #2967

Reason: Add a MiniMax OpenAI-compatible fixture that verifies both current models through Context7Agent at the configured regional endpoints.

• Add @ai-sdk/openai as a development dependency.
• Add a deterministic fixture test for MiniMax-M3 and MiniMax-M2.7 at the global and China endpoints.
• Verify that the Context7Agent path sends chat-completion requests with the selected model and Context7 tools.

Checks:

• pnpm install --frozen-lockfile --ignore-scripts
• pnpm --filter @upstash/context7-tools-ai-sdk typecheck
• pnpm --filter @upstash/context7-tools-ai-sdk exec vitest run src/minimax.test.ts
• ./node_modules/.bin/tsup from packages/tools-ai-sdk
• ESLint and Prettier checks for the changed files
• Secret scan passed
• The full package suite was also run; its five network-backed cases require an unset cloud-region environment variable, while the remaining fifteen tests passed.