Fix undefined array key in DeduplicationHandler with concurrent access (#2020)

When multiple requests write to the deduplication store file simultaneously,
partially written lines can be read by concurrent processes. This causes
'Undefined array key' errors when trying to destructure incomplete lines.

Changes:
- Skip incomplete/invalid lines in isDuplicate() instead of crashing
- Add LOCK_EX flag to file_put_contents() for atomic writes

Fixes #1950

Author: QDenka

src/Monolog/Handler/DeduplicationHandler.php

@@ -82,7 +82,7 @@ public function flush(): void
                 $passthru = $passthru === true || !\is_array($store) || !$this->isDuplicate($store, $record);
                 if ($passthru) {
                     $line = $this->buildDeduplicationStoreEntry($record);
-                    file_put_contents($this->deduplicationStore, $line . "\n", FILE_APPEND);
+                    file_put_contents($this->deduplicationStore, $line . "\n", FILE_APPEND | LOCK_EX);
                     if (!\is_array($store)) {
                         $store = [];
                     }
@@ -114,7 +114,14 @@ protected function isDuplicate(array $store, LogRecord $record): bool
         $yesterday = time() - 86400;
 
         for ($i = \count($store) - 1; $i >= 0; $i--) {
-            list($timestamp, $level, $message) = explode(':', $store[$i], 3);
+            $parts = explode(':', $store[$i], 3);
+
+            if (\count($parts) < 3) {
+                // Skip invalid/incomplete lines (e.g. partially written due to concurrent access)
+                continue;
+            }
+
+            [$timestamp, $level, $message] = $parts;
 
             if ($level === $record->level->getName() && $message === $expectedMessage && $timestamp > $timestampValidity) {
                 return true;