[Server] Add ClientGateway::supportsSampling() (#418)

Sampling was the one client capability a tool could not check before asking.
`supportsRoots()` and `supportsElicitation()` both read the capabilities the
client advertised during the handshake; sampling is stored in that same session
data but had no accessor, so a tool had to issue the request and catch the
refusal.

src/Server/ClientGateway.php

@@ -250,6 +250,23 @@ class ClientGateway
         return \array_key_exists('elicitation', $capabilities);
     }
 
+    /**
+     * Check if the connected client supports sampling.
+     *
+     * Sampling lets a server borrow the client's model during tool execution.
+     * This method checks the client's advertised capabilities to determine if
+     * sampling/createMessage requests are supported.
+     *
+     * @return bool True if the client supports sampling, false otherwise
+     */
+    public function supportsSampling(): bool
+    {
+        $capabilities = (array) $this->session->get('client_capabilities', []);
+
+        // MCP spec: capability presence indicates support (value is typically {} or [])
+        return \array_key_exists('sampling', $capabilities);
+    }
+
     /**
      * Send a request to the client and wait for a response (blocking).
      *