Add unit tests for getDefaultFilters in pkg/compose

[htoyoda18]

What I did
Added unit tests for the getDefaultFilters() function in pkg/compose/containers.go.

This function builds Docker API filter combinations and has conditional logic for handling oneOff modes and service selection.

Related issue

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[ndeloof]

compose test suite is mostly based on end-to-end tests, so we capture actual result when using the Moby API.

[htoyoda18]

compose test suite is mostly based on end-to-end tests, so we capture actual result when using the Moby API.

Thanks for the clarification! I now understand that the project prefers E2E tests over unit tests for internal helpers to ensure actual behavior via the Moby API.

I'll close this PR for now. I appreciate the guidance and look forward to contributing in a way that better aligns with the project's goals in the future!