Keep an attached certificate chain that carries no root

[Eljees]

Fixes #3976.

When a signature carries an attached certificate chain, the intermediate pool is built like this:

if len(chain) > 1 {
    if co.IntermediateCerts == nil {
        pool = x509.NewCertPool()
        for _, cert := range chain[:len(chain)-1] {
            pool.AddCert(cert)
        }
    }
}

Both parts assume the chain ends with a root. That holds when the signer attaches the whole chain, but not when the root CA is supplied separately — then every certificate in the chain is an intermediate. With a single intermediate attached, len(chain) > 1 is false, the pool is never built, and the attached intermediate is dropped entirely.

That is the asymmetry in the report. Running the current logic verbatim against generated certificates:

chain=[intermediate]        -> pool=nil        the attached intermediate is discarded
chain=[intermediate, root]  -> pool with 1 cert

Which matches both observations: attaching intermediate + root works (with the root duplicated), attaching the intermediate alone fails unless it is also passed on the command line.

The pool building moves into intermediatePoolFromChain, which drops the last element only when it is actually self-signed, and keeps it otherwise. Existing behaviour for chains that do end with a root is unchanged.

Covered by a table test over the five shapes: empty, intermediate only, intermediate + root, two intermediates, root only. pkg/cosign is green with the change and on a clean tree.

go build ./... could not finish here — the module proxy timed out fetching open-policy-agent/opa, unrelated to this change; go vet ./pkg/cosign/ and gofmt are clean.

AI-assisted (LLM used for drafting); the reproduction and runs above are mine.

[Hayden-IO]

Thank you for looking into this, but this code path will be removed as of #4959, so no need to fix these edge cases at this point.