rust volume: exit cleanly on SIGTERM, and make the harness notice when a server does not (#12370)

* rust volume: exit cleanly on SIGTERM and SIGINT

The shutdown path raced the HTTP and gRPC tasks with select!, then
awaited both handles again; tokio panics when a JoinHandle is polled after
it completed, so every signal ended in "JoinHandle polled after
completion" with exit status 101 and the volumes never closed. Waiting
for both tasks with join! is enough: both stop on the shutdown signal, and
a gRPC serve error sends it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test/volume_server: fail when a stopped server does not exit cleanly

The harness ignored the exit status after SIGINT, which is how the Rust
volume server's shutdown panic went unseen. Every server it stops must
now exit 0 within 10s. The Go volume servers get -preStopSeconds=0, which
the Rust ones already had, so a stop does not spend the default 10s grace
inside that window.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* test/volume_server: let assertArgPair find flags after a bare flag

rustVolumeArgs carries --volume.allowUntrustedRemoteEndpoints without a
value, so pairs no longer sit at even indexes and
TestRustVolumeArgsIncludeReadMode reported --readMode missing although it
was there.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011UpvafvK8JpagF94MHWqwK

* rust volume: stop both servers when either one exits

A gRPC task that panicked left the HTTP server and heartbeat running
until a signal arrived. Whichever server task ends first now sends the
shutdown and the other is awaited once, matching Go where a serve
failure is fatal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* rust volume: report an HTTP task panic as a server error

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

test/volume_server/framework/cluster_multi_rust.go

@@ -178,9 +178,9 @@ func (c *RustMultiVolumeCluster) Stop() {
 	c.cleanupOnce.Do(func() {
 		// Stop volume servers in reverse order
 		for i := len(c.volumeCmds) - 1; i >= 0; i-- {
-			stopProcess(c.volumeCmds[i])
+			stopProcess(c.testingTB, fmt.Sprintf("volume server %d", i), c.volumeCmds[i])
 		}
-		stopProcess(c.masterCmd)
+		stopProcess(c.testingTB, "master", c.masterCmd)
 		if !c.keepLogs && !c.testingTB.Failed() {
 			_ = os.RemoveAll(c.baseDir)
 		} else if c.baseDir != "" {