fix(events): keep earliest time on out-of-order buffer merges

Bring the implementation in line with the docblock/PR description
promise: when two collect() calls fold into the same buffer key with
different queued times, keep the earlier one. Previously the first
non-null time won regardless of value, so an out-of-order redelivery
could stamp the merged row with a later timestamp than the true
earliest one.

Author: lohanidamodar

src/Usage/Accumulator.php

@@ -95,9 +95,10 @@ public function collect(string $tenant, string $metric, int $value, string $type
         if ($type === Usage::TYPE_EVENT && isset($this->buffer[$key])) {
             // Additive: sum values for the same tenant + metric + tags combination.
             // Preserve the earliest queued time — later calls fold in without
-            // moving the bucket's timestamp forward.
+            // moving the bucket's timestamp forward, even when they arrive
+            // out of order (e.g. an earlier event redelivered after a later one).
             $this->buffer[$key]['value'] += $value;
-            if ($time !== null && !isset($this->buffer[$key]['time'])) {
+            if ($time !== null && (!isset($this->buffer[$key]['time']) || $time < $this->buffer[$key]['time'])) {
                 $this->buffer[$key]['time'] = $time;
             }
         } else {

tests/Usage/AccumulatorTest.php

@@ -329,6 +329,25 @@ public function testEventCollectPreservesEarliestQueuedTime(): void
         $this->assertSame($earlier, $entry['time']);
     }
 
+    public function testEventCollectKeepsEarliestTimeOnOutOfOrderMerge(): void
+    {
+        // Out-of-order redelivery: the later time arrives FIRST, the earlier
+        // time second. The merged bucket must carry the earlier time so
+        // buckets never slide forward when late-arriving events fold in.
+        $later = new \DateTime('2026-04-15 12:05:00');
+        $earlier = new \DateTime('2026-04-15 12:00:00');
+
+        $this->accumulator->collect('t1', 'requests', 10, Usage::TYPE_EVENT, [], $later);
+        $this->accumulator->collect('t1', 'requests', 5, Usage::TYPE_EVENT, [], $earlier);
+
+        $this->assertEquals(1, $this->accumulator->count());
+        $this->assertTrue($this->accumulator->flush());
+
+        $entry = $this->adapter->batches[0]['metrics'][0];
+        $this->assertEquals(15, $entry['value']);
+        $this->assertSame($earlier, $entry['time']);
+    }
+
     public function testGaugeCollectUsesLastWriteWinsForTime(): void
     {
         // Gauge collect() is last-write-wins on value; the queued time