[Server][Capability] Do not announce an externally loaded registry as changed (#490)

* [Capability] Do not announce an externally loaded registry as changed

`Registry` suppresses its `*ListChangedEvent`s while it is loading — `dispatch()`
returns early on the `loading` guard, so the elements a loader registers are the
registry's initial contents rather than a change to them.
`testListChangedEventsAreSuppressedDuringTheDeferredLoad` pins that.

The guard only covers `Registry::load()`, which needs the loader the constructor
took. A registry the caller built cannot be given one that way, so
`Builder::resolve()` loads it from the outside instead — by calling
`$chainLoader->load($registry)` directly, which never sets the guard. Every
element then dispatches on the way in.

With a notification bus configured that is not quiet. `PublishingEventDispatcher`
turns each event into a published notification, so every `build()` puts one
`list_changed` per element on the bus for a registry that did not change. Under
PHP-FPM, where the server is built per request and `Psr16NotificationBus` is
shared and persistent, every request broadcasts its whole element list to every
open `subscriptions/listen` stream and consumes the 256-entry backlog — after
which a reader that fell behind silently skips real notifications.

Split the guarded body of `load()` into `loadFrom(LoaderInterface $loader)` and
route the custom-registry branch through it.

`loadFrom()` takes only the `loading` guard, not the `loaded` bookkeeping, which
stays with `load()`. The loader it runs belongs to the caller, so it cannot stand
in for the one the registry was constructed with: marking the registry loaded
would retire a constructor loader that never ran, and would make a second
`loadFrom()` — one registry handed to two builders — a silent no-op. Keeping
`loaded` out of it also keeps `load()`'s promise that a transient failure is
retried on the next read, including when the failing loader reads the registry
during its own run.

`RegistryInterface` declares no `load()`, so this stays on the concrete
`Registry`, and a third-party implementation keeps the path it has today.

* Apply batched suggestions from code review

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

tests/Unit/Server/BuilderTest.php

@@ -15,6 +15,7 @@ use Mcp\Capability\Registry;
 use Mcp\Capability\Registry\ElementReference;
 use Mcp\Capability\Registry\Loader\LoaderInterface;
 use Mcp\Capability\Registry\ReferenceHandlerInterface;
+use Mcp\Capability\RegistryInterface;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Exception\LogicException;
 use Mcp\Schema\Content\TextContent;
@@ -32,6 +33,8 @@ use Mcp\Server\Handler\Request\InitializeHandler;
 use Mcp\Server\Protocol;
 use Mcp\Server\Session\SessionInterface;
 use Mcp\Server\Stateless\StatelessProtocol;
+use Mcp\Server\Subscription\InMemoryNotificationBus;
+use Mcp\Server\Subscription\PublishingEventDispatcher;
 use Mcp\Tests\Unit\Server\Extension\ThingExtension;
 use Mcp\Tests\Unit\Server\Extension\ThingListHandler;
 use Mcp\Tests\Unit\Server\Extension\ThingListRequest;
@@ -404,6 +407,42 @@ final class BuilderTest extends TestCase
 
         $this->fail('CallToolHandler not found in request handlers');
     }
+
+    public function testBuildingWithASuppliedRegistryDoesNotAnnounceTheLoadAsAChange(): void
+    {
+        // Otherwise every build publishes one list_changed per element, for no change.
+        $bus = new InMemoryNotificationBus();
+        $registry = new Registry(new PublishingEventDispatcher($bus));
+
+        Server::builder()
+            ->setRegistry($registry)
+            ->setNotificationBus($bus)
+            ->addTool(static fn (): string => 'ok', 'alpha')
+            ->addTool(static fn (): string => 'ok', 'beta')
+            ->build();
+
+        $this->assertSame(0, $bus->cursor());
+        $this->assertTrue($registry->hasTool('alpha'));
+        $this->assertTrue($registry->hasTool('beta'));
+
+        // A real change after the build is still published.
+        $registry->unregisterTool('alpha');
+
+        $this->assertSame(1, $bus->cursor());
+    }
+
+    public function testAThirdPartyRegistryIsStillLoadedThroughThePlainLoader(): void
+    {
+        $registry = $this->createMock(RegistryInterface::class);
+        $registry->expects($this->once())
+            ->method('registerTool')
+            ->with($this->callback(static fn (Tool $tool): bool => 'alpha' === $tool->name));
+
+        Server::builder()
+            ->setRegistry($registry)
+            ->addTool(static fn (): string => 'ok', 'alpha')
+            ->build();
+    }
 }
 
 /**