feat(synthetics): add syn-python-selenium-4.0 runtime (#31101)

### Issue # (if applicable)

Closes #30137.

### Reason for this change
Add new runtime.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Library_python_selenium.html#CloudWatch_Synthetics_runtimeversion-syn-python-selenium-4.0



### Description of changes
Add `syn-python-selenium-4.0` for synthetics runtime ENUM.



### Description of how you validated changes
Modify a unit test and an integ test to test the new runtime.



### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*