Allow pre-built instance handlers in element registration

[valeriudev]

Registering an element with a pre-built object instance as the handler throws at build time:

$foo = new Foo();
Server::builder()->addTool(handler: [$foo, 'bar'], name: 'foo')->build();
// Mcp\Exception\InvalidArgumentException: Invalid array handler format.
// Expected [ClassName::class, 'methodName'].  (HandlerResolver.php)

This is the only place that rejects an instance: the Handler type alias already declares array{0: object|string, 1: string}, getHandlerDescription() already formats object handlers, and the runtime invoker (ReferenceHandler) already dispatches them via its is_callable branch. HandlerResolver::resolve()'s guard simply required a string in slot 0, out of sync with that contract.

Fix: accept an object in slot 0 and normalize it to its class name for reflection — the same is_object($h[0]) ? $h[0]::class : $h[0] idiom getHandlerDescription() already uses. The guard can't be replaced by is_callable(), since is_callable([Class::class, 'instanceMethod']) is false and that lazy-instantiate form must keep working.

This unblocks handler objects with constructor dependencies that the container-less new $className() fallback cannot build.

Tests: added testResolvesValidInstanceArrayHandler; full unit suite, CS, and PHPStan are green.

Fixes #369