[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.

docs/client.md

@@ -792,7 +792,7 @@ $samplingCallback = new class implements SamplingCallbackInterface {
                 role: Role::Assistant,
                 content: new TextContent($response),
                 model: 'mock-llm',
-                stopReason: 'end_turn',
+                stopReason: 'endTurn',
             );
         } catch (\Throwable $e) {
             throw new SamplingException(