build(deps): bump openai from 2.6.1 to 2.8.1

Bumps [openai](https://github.com/openai/openai-python) from 2.6.1 to 2.8.1.
- [Release notes](https://github.com/openai/openai-python/releases)
- [Changelog](https://github.com/openai/openai-python/blob/main/CHANGELOG.md)
- [Commits](openai/openai-python@v2.6.1...v2.8.1)

---
updated-dependencies:
- dependency-name: openai
  dependency-version: 2.8.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -2,6 +2,6 @@ slack-bolt==1.26.0
 pytest
 ruff
 slack-cli-hooks==0.1.0
-openai==2.6.1
+openai==2.8.1
 anthropic==0.72.0
 google-cloud-aiplatform==1.124.0