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

Thank you for your contribution!