fix: restore Node 18 support for context7 mcp

[mturac]

Summary

• pin @upstash/context7-mcp runtime dependencies to Node 18-compatible major versions
• update the lockfile so Node 18 does not load undici 7 at startup

Closes #2761

Validation

• pnpm install --filter @upstash/context7-mcp --frozen-lockfile
• pnpm --filter @upstash/context7-mcp build
• pnpm --filter @upstash/context7-mcp typecheck
• pnpm --filter @upstash/context7-mcp test
• pnpm --filter @upstash/context7-mcp lint:check
• NPM_CONFIG_CACHE=/tmp/context7-node18-npm-cache npm exec --package node@18.20.8 -- node packages/mcp/dist/index.js --version