Add fingerprint capture cleanliness test for --artifact-type=oci

[AlexKantor87]

Background

PR #846 adds three tests in `cmd/kosli/fingerprint_capture_test.go` that pin the customer-facing contract for `kosli fingerprint`: stdout = fingerprint, stderr = empty, so that `FP=$(kosli fingerprint ... 2>&1)` works in CI. These tests cover `--artifact-type=file`, `--artifact-type=dir`, and (in a follow-up commit on the same PR) `--artifact-type=docker`.

`--artifact-type=oci` was deferred because there is no OCI-fingerprint test scaffolding in the repo today — `fingerprint_test.go` only covers `file`, `dir`, and `docker`. Adding an OCI test needs either a fake registry or a stable public OCI image.

This issue tracks adding the OCI variant so the contract is fully covered.

Why it matters

The `--artifact-type=oci` code path (`internal/server` → registry HTTP calls) is entirely separate from the file/dir/docker paths and could legitimately introduce stderr output (HTTP debug, registry authentication warnings, etc.) without anyone noticing — exactly the regression class server#5564 was about.

Suggested approach

Two reasonable options:

1. Use the existing `cli_registry` container in `docker-compose.yml`. Push a small known image to the local registry in a SetupSuite, then fingerprint it via OCI and assert capture cleanliness.
2. Use a stable public OCI image — e.g. `ghcr.io/distroless/static` — pinned by digest. Simpler, but introduces a network dep on test runs.

Option 1 matches the existing CI infrastructure better and is the recommended path.

Acceptance criteria

• A test in `cmd/kosli/fingerprint_capture_test.go` (or a new `fingerprint_capture_oci_test.go` if cleaner) that:
  • Stubs the version-check via `version.SetCheckForUpdateOverride`
  • Runs `kosli fingerprint --artifact-type=oci`
  • Asserts `stdout == "\n"`, `stderr == ""`
  • Asserts the `combined` (stdout+stderr) buffer parses as a fingerprint — the `2>&1` capture pattern
• Same shape as `TestFingerprintFile_CaptureCleanliness` for consistency

Context

• Original bug: kosli-dev/server#5564 "kosli-fingerprint is broken again"
• Fix that introduced the contract: PR #840
• Contract test that this builds on: PR #846
• Raised in PR review by @JonJagger