fix(fluxcd): refuse a flux-plunger drain deadline that leaves no time

A drain deadline of zero or less is already spent when the drain
starts, so flux-plunger would exit holding every uninstall fence
without listing one. The chart could produce one: its 40-second floor
was a literal, so raising the manager's stop bound or the margin before
SIGKILL, which the drain subtraction is already tied to, would leave the
floor rendering a non-positive deadline. The binary now refuses such a
flag at startup, before it reaches for a cluster, and the chart's floor
and the timings the README and the values comment state are held to
the same constants.

A whole number of a million seconds or more printed in exponent form
and was refused as a fraction; the range is now checked first, so it
gets the range message.

Assisted-by: LLM
Signed-off-by: Alexey Artamonov <aleksei.artamonov@aenix.io>
(cherry picked from commit 8bcb709ce8)

cmd/flux-plunger/main.go

@@ -82,6 +82,11 @@ func main() {
 
 	ctrl.SetLogger(zap.New(zap.UseFlagOptions(&opts)))
 
+	if err := fluxplunger.ValidateShutdownDrainTimeout(shutdownDrainTimeout); err != nil {
+		setupLog.Error(err, "invalid flag")
+		os.Exit(1)
+	}
+
 	// if the enable-http2 flag is false (the default), http/2 should be disabled
 	// due to its vulnerabilities. More specifically, disabling http/2 will
 	// prevent from being vulnerable to the HTTP/2 Stream Cancellation and