Refactor file extension extraction and consolidate test helpers

[Mbeaulne]

Description

Refactored file import functionality by extracting a reusable getFileExtension utility function and consolidating duplicate test helper functions. The getFileExtension function replaces inline file extension parsing logic in both OasisSubmitter and SubmitTaskArgumentsDialog components. Additionally, created a shared FileImportResult interface to unify the return types between CSV and JSON import functions, and moved common test helper functions (makeSecretArg and makeInput) to a dedicated testHelpers.ts file.

Type of Change

• Cleanup/Refactor

Checklist

• I have tested this does not break current pipelines / runs functionality
• I have tested the changes on staging

Test Instructions

1. Test file import functionality in the Oasis submitter by dragging and dropping CSV/JSON files
2. Verify that the import file button in the task arguments dialog works correctly
3. Ensure that file extension detection works properly for various file types
4. Run the existing test suites to confirm no regressions

Additional Comments

This refactoring improves code maintainability by eliminating duplicate logic and providing better type safety through shared interfaces. The changes are purely structural and should not affect the user-facing functionality.

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[Mbeaulne]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Refactor file extension extraction and consolidate test helpers #1896 👈 (View in Graphite)
• Add drag-and-drop file import and multi-format template export to OasisSubmitter #1895
• Add YAML file import support to task arguments dialog #1894
• Add JSON file import support to task arguments dialog #1892
• Add CSV import/export functionality to task arguments dialog #1890
• Add bulk submission support to OasisSubmitter with comma-separated input values #1889
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