Release Python SDK 8.1.0

[yaw-zebo]

Summary

• release response metadata envelope support from the SDK/OpenAPI update
• pin CI/release workflows to OpenAPI commit eba39c0d748252b2ad9e1e43ed56fede0cec35b8
• bump package version and changelog for the release

Validation

• OpenAPI: npm test
• TypeScript: npm run format:check && npm run lint && npm run typecheck && npm test && npm run build
• Go: gofmt check, GOWORK=off go vet ./..., GOWORK=off go test ./...
• Python: python3 -m unittest discover -s tests -p test_*.py
• Ruby: bundle exec rake test sorbet
• PHP: composer validate --strict --no-check-publish && vendor/bin/phpstan analyse --no-progress --memory-limit=512M && vendor/bin/phpunit
• .NET and Java local release checks were not run because dotnet, mvn, and mvnw are unavailable in this checkout; GitHub Actions must validate those release artifacts.