[WIP] Refactor GitLab scan to integrate with other scan commands

[Copilot]

Thanks for the feedback on #378. I've created this new PR, which merges into #378, to address your comment. I will work on the changes and keep this PR's description up to date as I make progress.

Original PR: #378
Triggering comment (#378 (comment)):

@copilot You are Copilot working on CompassSecurity/pipeleak (Go). Your goal: find packages under src/pipeleak that lack meaningful unit tests and add only meaningful *_test.go files next to those packages.

Do this concisely:

Scope: src/pipeleak only. Do NOT edit e2e tests or files outside src/pipeleak.
Test criteria: add tests that exercise real logic — error paths, edge cases, parsing, filtering, archive handling, API translation. Do NOT add trivial getters/setters tests.
Conventions: Go table-driven tests, use testify/assert, t.Parallel() where safe, t.TempDir() for IO, mock external clients via interfaces (fakes in _test.go), deterministic and fast.
Implementation: for each package produce (1) a short list of target functions/methods and why a test is needed, then (2) add *_test.go files covering normal, edge, and error cases. Use minimal production-code changes only if absolutely needed for testability (add tiny adapter interfaces).
Verification: run golangci-lint and go test ./... (excluding tests/e2e). Include test results in PR/commit message.
Deliverable: added test files, one-line rationale per file, and commands+results.

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