add test case and improved coverage for datasource/file/s3

[gourish25]

Pull Request Template

Description:

• This PR closes Unit Test for s3 in files/datasource #2442
• Added comprehensive test coverage for S3 file operations in file_test.go
• Added test coverage for S3 file system operations in fs_test.go
• Implemented unit tests for S3File methods including Read, Write, Seek, Close, and metadata operations
• Added tests for FileSystem methods including Connect, Create, Open, and directory operations

Additional Information:
Tests use mock objects for S3 client
Testify assertions used for better test readability
Helper functions created for consistent test setup

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.
• 87.0% test coverage maintained

Thank you for your contribution!

[Umang01-hash]

• Please remove all if-else conditions from the test methods and break them into smaller methods if needed for success and error cases.