fix(pipelines): codeStar connection accepts nested repository (#29631)

### Issue # (if applicable)

Closes #27504 

### Reason for this change

The current validation of `repoString` in constructor of `CodeStarConnectionSource` does not support nested repository that can appear in GitLab.

### Description of changes

The validation is fixed to accept nested repository.

### Description of how you validated changes

I added unit tests and confirmed all tests passed.

### 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*