Flat map headers for PSR request context

The PSR request headers come in a weird format. We'll flatten the array to a simple key-value format

src/Context/RequestContextProvider.php

@@ -120,7 +120,15 @@ public function getCookies(): array
      */
     public function getHeaders(): array
     {
-        return $this->request->headers->all();
+        /** @var array<string, list<string|null>> $headers */
+        $headers = $this->request->headers->all();
+
+        return array_filter(
+            array_map(
+                fn(array $header) => $header[0],
+                $headers
+            )
+        );
     }
 
     /**