[Server] Allow pre-built instance handlers in element registration (#375)

* Allow pre-built instance handlers in element registration

`HandlerResolver::resolve()` rejected `[$instance, 'methodName']` array
handlers even though the `Handler` type alias and the runtime invoker
already support them, so `Server::builder()->addTool(handler: [$obj, 'bar'])`
threw "Invalid array handler format" at build time.

Relax the array-shape guard to accept an object in slot 0 and normalize it
to its class name for reflection (the same idiom getHandlerDescription()
already uses). This unblocks dependency-injected handler objects that the
container-less `new $className()` fallback cannot build.

Fixes #369

* Use 0.6.1 changelog heading and test instance handler end-to-end

This is a bugfix with no new public API or BC break, so the changelog
entry belongs under 0.6.1 rather than a new 0.7.0 section.

Add an end-to-end test that registers a pre-built instance handler with a
constructor dependency the container-less new $className() fallback cannot
satisfy, then drives a real tools/call through CallToolHandler and asserts
the injected instance is the one invoked.

* Address review: reject Closure array handlers, fix abstract-method message

- HandlerResolver: extract the array-handler guard into a readable
  $hasValidTarget check that accepts strings and objects but excludes
  \Closure, so [$closure, 'method'] falls through to the format error
  instead of the misleading "method not found in class Closure".
- Fix the abstract-method rejection message ("must be abstract" ->
  "must not be abstract"), a pre-existing typo in the same branch.
- Tighten the CHANGELOG entry wording.
- Cover Closure rejection with a regression test.

* Move instance-handler entry under 0.7.0 changelog heading

Per review: registering pre-built instance handlers is a feature, not a
patch, so it belongs in a minor release heading.

tests/Unit/Server/BuilderTest.php

@@ -83,6 +83,25 @@ final class BuilderTest extends TestCase
         $this->assertSame('intercepted', $result);
     }
 
+    #[TestDox('A pre-built instance handler with constructor dependencies is registered and invoked on that instance')]
+    public function testPreBuiltInstanceHandlerIsInvokedOnTheGivenInstance(): void
+    {
+        // The handler's constructor requires an argument the container-less
+        // `new $className()` fallback can never satisfy. If the tool call
+        // succeeds, it can only be because the pre-built instance — carrying
+        // its injected dependency — was the one invoked.
+        $handler = new GreetingService('World');
+
+        $server = Server::builder()
+            ->setServerInfo('test', '1.0.0')
+            ->addTool(handler: [$handler, 'greet'], name: 'greet', description: 'Greets using the injected name')
+            ->build();
+
+        $result = $this->callTool($server, 'greet');
+
+        $this->assertSame('Hello, World', $result);
+    }
+
     #[TestDox('enableExtension() registers an extension and announces its capability payload')]
     public function testEnableExtensionRegistersExtension(): void
     {
@@ -166,3 +185,20 @@ final class BuilderTest extends TestCase
         $this->fail('CallToolHandler not found in request handlers');
     }
 }
+
+/**
+ * A handler whose constructor dependency cannot be satisfied by the
+ * container-less `new $className()` fallback, so it must be registered as a
+ * pre-built instance.
+ */
+final class GreetingService
+{
+    public function __construct(private string $name)
+    {
+    }
+
+    public function greet(): string
+    {
+        return 'Hello, '.$this->name;
+    }
+}