fix(watchdog): pin the heartbeat telemetry fixes with stronger tests

Review follow-up to the bandwidth/rps/producers fix. The nginx stub_status
parse moves into SystemInfo::nginxRequestCount(), so the requests field the
watchdog samples is tested against a real status body and an unreachable
one (no count, rps 0). The getStats() test now pins the whole array_replace
statement (a `+` would leave the bandwidth keys at 0), and the sampler test
checks that 120 s is in range by default. No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQHeypQpfAXTdCysmi7b6a

tests/Unit/SystemInfoNginxStatusTest.php

@@ -0,0 +1,27 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+use XcVm\Core\Util\SystemInfo;
+
+/**
+ * SystemInfo::nginxRequestCount — the counter the watchdog samples for
+ * servers.requests_per_second. stub_status reports accepts, handled and
+ * requests on its third line; reading the wrong field would feed a connection
+ * count into the request rate.
+ */
+final class SystemInfoNginxStatusTest extends TestCase {
+
+	public function testTheRequestCountIsTheThirdCounter(): void {
+		$rStatus = "Active connections: 3 \nserver accepts handled requests\n 10 10 57 \nReading: 0 Writing: 1 Waiting: 2 \n";
+
+		$this->assertSame(57.0, SystemInfo::nginxRequestCount($rStatus));
+	}
+
+	public function testAnUnreachableOrMalformedStatusHasNoCount(): void {
+		// The watchdog passes '' when the status page cannot be fetched.
+		$this->assertNull(SystemInfo::nginxRequestCount(''));
+		$this->assertNull(SystemInfo::nginxRequestCount("Active connections: 3 \nserver accepts handled requests\n"));
+		$this->assertNull(SystemInfo::nginxRequestCount("Active connections: 3 \nserver accepts handled requests\n 10 10 \n"));
+		$this->assertNull(SystemInfo::nginxRequestCount("<html>\n<head><title>404 Not Found</title></head>\n<body>x y z</body>\n"));
+	}
+}