Add vitest-pool-bun CI: run demo suite and selected Node unit tests in Bun pool (PoC)

[Copilot]

Summary

The vitest-pool-bun proof-of-concept package had no CI coverage. This PR adds a dedicated workflow that now validates both the PoC example suite and a selected subset of repository Node unit tests using the Bun pool runner.

• New workflow .github/workflows/vitest-pool-bun.yml, path-scoped to vitest-pool-bun/** (and the workflow file), triggered on push to main, pull_request, and workflow_dispatch.
• Keeps a pinned, least-privilege setup (permissions: {}, SHA-pinned actions, concurrency cancel-in-progress).
• Runs npm run demo in vitest-pool-bun (green PoC suite), then installs root dependencies and runs selected Node unit tests through Bun pool using:
  • vitest-pool-bun/examples/clickhouse-node-unit.config.mjs
  • packages/client-node/__tests__/unit/node_user_agent.test.ts
  • packages/client-node/__tests__/unit/node_default_logger.test.ts

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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