fix(tests): update file URI normalization for Windows compatibility

[Lee-WonJun]

#5958 issue

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Description

In FileUri Tests, the normalizeFileUri helper hardcoded the Windows drive letter as C:.
This caused test failures when running on a different drive (e.g., F:, D:).
This change updates the function to dynamically detect the current drive letter at runtime and use it when constructing the expected file URI.
The fix applies only to the test utility and does not affect production code.

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes
• A short description of the change has been added to the [CHANGELOG](https://github.com/aws/aws-toolkit-jetbrains/blob/master/CONTRIBUTING.md#contributing-via-pull-requests) if the change is customer-facing in the IDE.
• I have added metrics for my changes (if required)

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.