test(hooks): skip the stdin-JSON hook test when jq is missing

[dwin-gharibi]

printStdinJSONFieldCmd shells out to jq. When jq is absent the hook still runs and exits
cleanly, producing no output, so the test fails on its content assertion rather than on a
missing command — reporting a hook-plumbing defect that does not exist.

Closes #4052.

Before

On a machine without jq (e.g. stock golang:1.27):

--- FAIL: TestExecuteStopReceivesResponseContent (0.00s)
    hooks_test.go:586:
        	Error: "" does not contain "final answer content"
FAIL	github.com/docker/docker-agent/pkg/hooks

After

    hooks_test.go:573: jq is not installed; it is required to read a JSON field from hook stdin
--- SKIP: TestExecuteStopReceivesResponseContent (0.00s)
ok  	github.com/docker/docker-agent/pkg/hooks

With jq present, unchanged:

--- PASS: TestExecuteStopReceivesResponseContent (0.00s)
ok  	github.com/docker/docker-agent/pkg/hooks

What changed

The helper takes *testing.T and guards on exec.LookPath:

func printStdinJSONFieldCmd(t *testing.T, field string) string {
	t.Helper()
	if _, err := exec.LookPath("jq"); err != nil {
		t.Skip("jq is not installed; it is required to read a JSON field from hook stdin")
	}
	return `cat | jq -r '.` + field + `'`
}

The Windows mirror takes the same parameter for signature parity but needs no guard —
PowerShell's ConvertFrom-Json is built in. One caller updated.

Why skip rather than drop the dependency

Removing jq would mean either parsing JSON with sed/awk — fragile, and a fragile test is
worse than a skipped one — or weakening the assertion to "the value appears somewhere in the
payload", which stops testing that the value arrives in the named field.

Skipping keeps the assertion exact wherever jq exists (CI, most dev machines) and makes the
failure mode honest everywhere else. Coverage is unchanged on any machine that had jq.

Verification

Tested both paths in a container, by moving /usr/bin/jq aside and back:

	jq present	jq absent
before	ok	FAIL — misleading assertion
after	ok (PASS)	ok (SKIP, names jq)

Full pkg/hooks package passes in both states; go vet clean.

Note

This is a test-only change. No production code is touched, and behaviour on machines with jq
installed is byte-for-byte identical.

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