[Schema] Serialize a settingless extension as an object (#438)

* [Schema] Serialize a settingless extension as an object

An extension declaring support with no settings was advertised as `[]`, an
empty JSON array, which the wire schema rejects - `extensions` maps an id to
a settings *object*. Both ServerCapabilities and ClientCapabilities now cast
each entry, so it goes out as `{}`.

* Apply suggestion from @chr-hertel

src/Schema/ClientCapabilities.php

@@ -188,7 +188,10 @@ class ClientCapabilities implements \JsonSerializable
         }
 
         if ($this->extensions) {
-            $data['extensions'] = (object) $this->extensions;
+            $data['extensions'] = (object) array_map(
+                static fn (mixed $settings): mixed => \is_array($settings) ? (object) $settings : $settings,
+                $this->extensions,
+            );
         }
 
         return $data ?: new \stdClass();

src/Schema/ServerCapabilities.php

@@ -189,7 +189,12 @@ class ServerCapabilities implements \JsonSerializable
         }
 
         if ($this->extensions) {
-            $data['extensions'] = (object) $this->extensions;
+            // Each entry is a settings *object*; an extension with no settings
+            // declares `{}`, and an empty PHP array would serialize as `[]`.
+            $data['extensions'] = (object) array_map(
+                static fn (mixed $settings): mixed => \is_array($settings) ? (object) $settings : $settings,
+                $this->extensions,
+            );
         }
 
         return $data;