Add unit tests for findUniqueFileName

[Kota-Jagadeesh]

Description (required)

Fixes #5923

What changes did you make and why?

• the logic update: Modified the findUniqueFileName in UploadWorker.kt to replace the old 5-digit random hash format (12345) with the requested 3-digit hash format #XXX.
• test implementation: Created UploadWorkerTest.kt to ensure the filename generation logic is robust.
• visibility vhange: updated findUniqueFileName visibility to internal to allow unit test access while maintaining module-level encapsulation.

Why?
The previous 5-digit format did not meeet the updated project requirements. these changes align the naming convention with the #XXX standard and provide automated testing to ensure filenames are generated correctly and remain unique.

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Tests performed (required)

Tested ProdDebug build variant on Local Unit Tests (JUnit 4).

[github-actions]

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