[Server] Fix: standardize error handling across handlers with MCP specificatio… (#124)

* fix: standardize error handling across handlers with MCP specification compliance

* Update mcp-elements.md

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

* docs: clarify error handling behavior for all element types

* docs: remove unnecessary whitespace in mcp-elements.md

* fix: simplify exception signatures to accept simple messages

* refactor: use specific exceptions in examples for better error handling

* feat: throw exceptions from reference provider instead of returning null

---------

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

src/Schema/JsonRpc/Error.php

@@ -106,6 +106,11 @@ class Error implements MessageInterface
         return new self($id, self::SERVER_ERROR, $message);
     }
 
+    public static function forResourceNotFound(string $message, string|int $id = ''): self
+    {
+        return new self($id, self::RESOURCE_NOT_FOUND, $message);
+    }
+
     public function getId(): string|int
     {
         return $this->id;