[Schema][Server] Harden sampling tools against the spec (#420)

* harden sampling tools against the spec

Report tool-flow violations as -32602 instead of dropping the request,
accept resource_link in tool results, gate tools on sampling.tools, and
reject empty content.

* normalize keyed content arrays to lists

array_filter() and friends preserve keys, and a keyed array serializes as a
JSON object instead of the content-block array the schema requires.

examples/client/http_client_communication.php

@@ -61,7 +61,7 @@ $samplingRequestHandler = new SamplingRequestHandler(new class implements Sampli
             role: Role::Assistant,
             content: new TextContent($mockResponse),
             model: 'mock-gpt-4',
-            stopReason: 'end_turn',
+            stopReason: 'endTurn',
         );
     }
 });

examples/client/stdio_client_communication.php

@@ -50,7 +50,7 @@ $samplingRequestHandler = new SamplingRequestHandler(new class implements Sampli
             role: Role::Assistant,
             content: new TextContent($mockResponse),
             model: 'mock-gpt-4',
-            stopReason: 'end_turn',
+            stopReason: 'endTurn',
         );
     }
 });