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

[chr-hertel]

Registry suppresses its *ListChangedEvents while loading, but the loading guard is only set by Registry::load(). Builder::resolve() loads a caller-supplied registry by calling $chainLoader->load($registry) directly, which bypasses it — so with a notification bus configured, every build() publishes one list_changed per element for a registry that did not change.

$registry = new Registry(new PublishingEventDispatcher($bus));
$tools(Server::builder()->setRegistry($registry)->setNotificationBus($bus))->build();
echo $bus->cursor();   // 3, and 0 for a registry the Builder constructs itself

InMemoryNotificationBus mostly gets away with it. Psr16NotificationBus does not: under PHP-FPM the server is built per request and the bus is shared, so every request broadcasts its whole element list to every open subscriptions/listen stream and consumes the 256-entry backlog — after which a lagging reader silently skips real notifications.

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

loadFrom() takes only the loading guard; loaded stays with load(). The loader it runs is the caller's, so marking the registry loaded there would retire a constructor loader that never ran, and make a second loadFrom() — one registry handed to two builders — a silent no-op.

RegistryInterface declares no load(), so this stays on the concrete Registry; anything else keeps the path it has today. No BC break.

Found in symfony/mcp-bundle, which always supplies its own registry (symfony/ai#2458).