Publish tiny stdio testing export files

[yanziwei]

Summary

• remove the package files exclusion that prevented dist/testing.js and dist/testing.d.ts from being published
• keeps the documented tiny-stdio-mcp-server/testing subpath export resolvable in package tarballs

Verification

• npm run build --workspaces=false from packages/tiny-stdio-mcp-server
• verified dist/testing.js and dist/testing.d.ts are generated
• npm pack --dry-run --json confirms both testing export targets are included
• git diff --check