[Schema] Omit the id of an error response that never had one (#442)

* [Schema] Omit the id of an error response that never had one

An error response whose id could not be read sent `"id": ""`, which claims
the peer issued a request with an empty-string id - a different statement
from "the id could not be read". JSON-RPC spells the latter as a null id,
and a receiver that cannot read the id is exactly the case the code covers.

[BC Break] Error::$id accepts null and getId() may return it. All the for*()
factories default to null, fromArray() accepts a missing or explicitly-null
id, and the member is omitted from the serialized form when absent.

* Guard null response id in Client/Server Protocol handlers

Also fix Error::jsonSerialize()'s phpdoc: data belongs inside error, not beside it.

tests/Unit/Client/ProtocolTest.php

@@ -99,6 +99,19 @@ final class ProtocolTest extends TestCase
         yield 'modern revision' => [ProtocolVersion::V2026_07_28->value];
     }
 
+    #[TestDox('logs and ignores an id-less error response instead of crashing on it')]
+    public function testIgnoresIdLessErrorResponse(): void
+    {
+        $protocol = new Protocol(logger: $logger = new CollectingLogger());
+
+        $protocol->processMessage(json_encode([
+            'jsonrpc' => MessageInterface::JSONRPC_VERSION,
+            'error' => ['code' => -32700, 'message' => 'Parse error'],
+        ], \JSON_THROW_ON_ERROR));
+
+        $this->assertCount(1, $logger->warnings);
+    }
+
     private function createConfiguration(ProtocolVersion $protocolVersion): Configuration
     {
         return new Configuration(