[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.

docs/client.md

@@ -600,6 +600,51 @@ Only the `Accept` action carries content.
 See `examples/client/stdio_elicitation.php` for a runnable example against the
 elicitation demo server.
 
+### Roots
+
+Roots let the client expose a list of `file://` "workspace folders" that the server
+is allowed to operate on. Advertise the `roots` capability and register a handler
+that answers server `roots/list` requests:
+
+```php
+use Mcp\Client\Handler\Request\ListRootsRequestHandler;
+use Mcp\Client\Handler\Request\RootsCallbackInterface;
+use Mcp\Schema\ClientCapabilities;
+use Mcp\Schema\Request\ListRootsRequest;
+use Mcp\Schema\Result\ListRootsResult;
+use Mcp\Schema\Root;
+
+class WorkspaceRootsCallback implements RootsCallbackInterface
+{
+    public function __invoke(ListRootsRequest $request): ListRootsResult
+    {
+        return new ListRootsResult([
+            new Root('file:///home/user/projects/app', 'Application'),
+            new Root('file:///home/user/projects/library', 'Library'),
+        ]);
+    }
+}
+
+$client = Client::builder()
+    ->setCapabilities(new ClientCapabilities(roots: true, rootsListChanged: true))
+    ->addRequestHandler(new ListRootsRequestHandler(new WorkspaceRootsCallback))
+    ->build();
+```
+
+When the client's roots change, notify the server so it can request the updated
+list via `roots/list`. This requires advertising the `roots.listChanged`
+capability (`rootsListChanged: true` above); otherwise `sendRootsListChanged()`
+throws a `RuntimeException`. On a client that is not connected it throws a
+`ConnectionException`:
+
+```php
+$client->sendRootsListChanged();
+```
+
+See `examples/client/stdio_roots.php` for a runnable example: it calls the
+`inspect_workspace_roots` tool of the client-communication demo server, which
+answers by issuing the `roots/list` request back to the client.
+
 ## Error Handling
 
 The client throws exceptions for various error conditions: