[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;

tests/Unit/Schema/Extension/CapabilitiesExtensionsTest.php

@@ -57,6 +57,24 @@ class CapabilitiesExtensionsTest extends TestCase
         $this->assertObjectHasProperty(McpApps::EXTENSION_ID, $json['extensions']);
     }
 
+    public function testServerCapabilitiesSerializeSettinglessExtensionAsObject(): void
+    {
+        $caps = new ServerCapabilities(tools: true, extensions: ['acme/thing' => []]);
+
+        $json = json_encode($caps->jsonSerialize(), \JSON_UNESCAPED_SLASHES);
+
+        $this->assertStringContainsString('"acme/thing":{}', (string) $json);
+    }
+
+    public function testClientCapabilitiesSerializeSettinglessExtensionAsObject(): void
+    {
+        $caps = new ClientCapabilities(elicitation: true, extensions: ['acme/thing' => []]);
+
+        $json = json_encode($caps->jsonSerialize(), \JSON_UNESCAPED_SLASHES);
+
+        $this->assertStringContainsString('"acme/thing":{}', (string) $json);
+    }
+
     public function testServerCapabilitiesJsonSerializeWithoutExtensions(): void
     {
         $caps = new ServerCapabilities(tools: true);