hsic: add ReadLog method for container log inspection

Add ReadLog() to HeadscaleInContainer and the ControlServer interface
so integration tests can inspect container logs at runtime. This is
needed by the SSH check mode tests to extract the auth-id from
headscale log output.

Updates #1850

integration/control.go

@@ -16,6 +16,7 @@ import (
 type ControlServer interface {
 	Shutdown() (string, string, error)
 	SaveLog(path string) (string, string, error)
+	ReadLog() (string, string, error)
 	SaveProfile(path string) error
 	Execute(command []string) (string, error)
 	WriteFile(path string, content []byte) error

integration/hsic/hsic.go

@@ -699,6 +699,18 @@ func (t *HeadscaleInContainer) WriteLogs(stdout, stderr io.Writer) error {
 	return dockertestutil.WriteLog(t.pool, t.container, stdout, stderr)
 }
 
+// ReadLog returns the current stdout and stderr logs from the headscale container.
+func (t *HeadscaleInContainer) ReadLog() (string, string, error) {
+	var stdout, stderr bytes.Buffer
+
+	err := dockertestutil.WriteLog(t.pool, t.container, &stdout, &stderr)
+	if err != nil {
+		return "", "", fmt.Errorf("reading container logs: %w", err)
+	}
+
+	return stdout.String(), stderr.String(), nil
+}
+
 // SaveLog saves the current stdout log of the container to a path
 // on the host system.
 func (t *HeadscaleInContainer) SaveLog(path string) (string, string, error) {