Update python-dotenv requirement in /samples/durable-task-sdks/python (#302)

Updates the requirements on [python-dotenv](https://github.com/theskumar/python-dotenv) to permit the latest version.
- [Release notes](https://github.com/theskumar/python-dotenv/releases)
- [Changelog](https://github.com/theskumar/python-dotenv/blob/main/CHANGELOG.md)
- [Commits](theskumar/python-dotenv@v1.0.0...v1.2.2)

---
updated-dependencies:
- dependency-name: python-dotenv
  dependency-version: 1.2.2
  dependency-type: direct:production
...

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samples/durable-task-sdks/python/arXiv_research_agent/requirements.txt

@@ -17,7 +17,7 @@ uvicorn>=0.27.0
 httpx>=0.26.0
 
 # Environment variables
-python-dotenv>=1.0.0
+python-dotenv>=1.2.2
 
 # Rich console output (optional, for CLI)
 rich>=15.0.0