[Client][Server] Add Roots support (#395)

* [Client][Server] Add Roots support

Wire up the MCP "roots" capability on both sides:

Client (php-sdk as client):
- RootsCallbackInterface + ListRootsRequestHandler answer server roots/list
  requests, mirroring the Sampling handler.
- Client::sendRootsListChanged() emits notifications/roots/list_changed.

Server (php-sdk as server):
- ClientGateway::listRoots() requests the client's roots, and
  supportsRoots() reports whether the client advertised the capability,
  normalizing the stored capabilities to an array so an empty
  capabilities object (\stdClass) doesn't cause a TypeError.
- Add ListRootsResult::fromArray() (used by listRoots()).
- ListRootsRequestHandler forwards RootsException messages to the server
  (new Mcp\Exception\RootsException), mirroring SamplingRequestHandler.
- Client::sendRootsListChanged() throws when the client did not advertise
  the roots.listChanged capability.

Examples, docs and tests:
- examples/README/docs advertise
  `new ClientCapabilities(roots: true, rootsListChanged: true)` before
  sending notifications/roots/list_changed.
- CHANGELOG entry for Roots (unreleased 0.7.0 section at the time).
- Unit tests for capability serialization/round-trip, sendRootsListChanged
  gating, and RootsException message forwarding.

phpunit, php-cs-fixer and phpstan green.

* [Client][Server] Address review: schema validation, changelog section, connection guard, working example

Responds to chr-hertel's review of the Roots support:

- ListRootsResult::fromArray() rejects non-array root elements instead of
  accepting malformed entries silently.
- CHANGELOG entry moved to the next unreleased section as the release
  train advanced past it (0.7.0 -> 0.8.0).
- Client::sendRootsListChanged() went straight to
  Protocol::sendNotification(), which sends via
  `$this->transport?->send()` -- on a client that is not connected the
  notification was silently dropped. It now throws a ConnectionException
  like sendRequest() does.
- The stdio example only advertised the roots capability, so the
  roots/list handler never actually ran. The client-communication demo
  server gets an `inspect_workspace_roots` tool that calls
  ClientGateway::supportsRoots()/listRoots(); the example calls it, so
  the server actually issues roots/list and the client answers it.
  Dropped the try/catch wrapper from the example.
- ClientTest's connected-client mock now goes through setState() so
  isConnected() reflects a real handshake instead of a stubbed flag.

phpunit, php-cs-fixer and phpstan green.

examples/server/client-communication/ClientAwareService.php

@@ -25,6 +25,42 @@ final class ClientAwareService
         $this->logger->info('SamplingTool instantiated for sampling example.');
     }
 
+    /**
+     * Ask the client which workspace folders the server is allowed to operate on.
+     *
+     * Demonstrates the server side of the "roots" client capability: the tool
+     * issues a roots/list request that the client answers from its own handler.
+     *
+     * @return array{status: string, message: string, roots?: list<array{uri: string, name: string|null}>}
+     */
+    #[McpTool(name: 'inspect_workspace_roots', description: 'Ask the client for its workspace roots via a roots/list request.')]
+    public function inspectWorkspaceRoots(RequestContext $context): array
+    {
+        $clientGateway = $context->getClientGateway();
+
+        if (!$clientGateway->supportsRoots()) {
+            return [
+                'status' => 'unsupported',
+                'message' => 'Client does not expose roots. Advertise the "roots" capability and register a ListRootsRequestHandler to let the server discover your workspace folders.',
+            ];
+        }
+
+        $result = $clientGateway->listRoots();
+
+        $roots = [];
+        foreach ($result->roots as $root) {
+            $roots[] = ['uri' => $root->uri, 'name' => $root->name];
+        }
+
+        $clientGateway->log(LoggingLevel::Info, \sprintf('Client exposed %d root(s).', \count($roots)));
+
+        return [
+            'status' => 'ok',
+            'message' => \sprintf('Client exposed %d root(s).', \count($roots)),
+            'roots' => $roots,
+        ];
+    }
+
     /**
      * @return array{incident: string, recommended_actions: string, model: string}
      */