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Add security vulnerability checks using `bandit` and `safety`.

* **Dependencies**: Add `bandit` and `safety` as development dependencies in `pyproject.toml`.
* **Security Check Script**: Create `droidrun/tools/security_check.py` to run `bandit` and `safety`.
* **Documentation**: Update `README.md` and `CONTRIBUTING.md` with instructions to run security checks.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/droidrun/droidrun?shareId=XXXX-XXXX-XXXX-XXXX).