test(patch): add single-image helper coverage and fix EnsurePath error handling

[omercnet]

Part of the coverage-improvement series tracked in #1580.

Summary

Adds focused unit coverage for single-image patch helper paths and fixes a real utils.EnsurePath bug exposed by those tests.

• Adds pkg/patch/single_test.go with focused coverage for pre-BuildKit single-image patch helpers.
• Extends pkg/patch/patch_validation_test.go with the unique parsePkgTypes cases from the first draft, avoiding duplicate validation-test ownership.
• Fixes pkg/utils.EnsurePath to return non-ENOENT os.Stat errors directly instead of dereferencing a nil FileInfo.

Why

pkg/patch/single.go is one of the largest low-coverage files in the package. Most of the full patching path requires live BuildKit/image infrastructure, but many guardrail/helper branches are unit-testable and should not require the integration suite.

While adding error-path tests for setupWorkingFolder, a bug in utils.EnsurePath surfaced: if os.Stat(path) returned an error that was not os.ErrNotExist, the old implementation entered the else branch and called st.IsDir() even though st was nil. The fix now returns non-ENOENT stat errors directly.

Covered areas

• Invalid image-reference guardrail in patchSingleArchImage
• validatePlatformEmulation host-platform happy path
• setupWorkingFolder temp, existing, explicit-dir, and error paths
• image reference resolution
• loader auto-detection and loader-creation error cleanup
• createPatchResultWithStates invalid patched-ref path
• package-type inclusion helpers
• report-parse and no-updates-after-filtering pre-BuildKit paths
• EnsurePath non-ENOENT stat error behavior

Test plan

go test ./pkg/patch/... ./pkg/utils/... -count=1
go vet ./pkg/patch/... ./pkg/utils/...
golangci-lint run --timeout=2m ./pkg/patch/... ./pkg/utils/...

All pass locally.

[Copilot]

Pull request overview

Adds focused unit-test coverage for single-arch patching helpers in pkg/patch/single.go, complementing existing integration coverage and improving Codecov-reported unit coverage without changing production behavior.

Changes:

• Add pkg/patch/single_test.go with table-driven unit tests for multiple helper/error paths in single.go.
• Cover working folder setup/cleanup behavior, reference parsing/resolution, loader selection, and pkg-type parsing/validation.

[omercnet]

@sozercan thanks for the EnsurePath fix on this branch — it looks like a real bug my tests exposed and I'm glad you patched it here.

One housekeeping issue: commit c67907a3 is missing a DCO trailer, so the PR now fails the DCO check. Could you please amend/sign off that commit and force-push?

Expected trailer format:

Signed-off-by: Sertac Ozercan <sozercan@gmail.com>

I intentionally didn't rewrite your commit from my side so authorship/signoff stays with you.

[omercnet]

I rewrote the branch cleanly on current main with one signed commit (c8969586) containing both the single-image helper tests and the EnsurePath fix. That removes the unsigned c67907a3 commit while preserving its behavior, so DCO should clear on the new run. Thanks again for catching/fixing the EnsurePath path — the new test coverage keeps it guarded.