refactor: put formatters into dedicated classes (#224)

Split formatting logic from Reference classes into dedicated formatter classes:
- ToolResultFormatter: Formats tool execution results into Content items
- PromptResultFormatter: Formats prompt results into PromptMessage items
- ResourceResultFormatter: Formats resource results into ResourceContents items

This refactor:
- Eliminates 122 lines of code duplication between ResourceReference and
  ResourceTemplateReference (identical formatting logic)
- Improves code organization by separating formatting concerns from
  capability reference management
- Makes formatters independently testable and reusable
- Reduces Reference classes by ~450 lines while maintaining all functionality

Changes:
- Create src/Capability/Formatter/* with three formatter classes
- Update Reference classes to delegate to formatters instead of inline logic
- Add comprehensive test coverage for all formatter classes
- All existing tests pass; no behavioral changes

tests/Unit/Capability/Formatter/PromptResultFormatterTest.php

@@ -0,0 +1,49 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Unit\Capability\Formatter;
+
+use Mcp\Capability\Formatter\PromptResultFormatter;
+use Mcp\Schema\Content\PromptMessage;
+use Mcp\Schema\Content\TextContent;
+use Mcp\Schema\Enum\Role;
+use PHPUnit\Framework\TestCase;
+
+class PromptResultFormatterTest extends TestCase
+{
+    public function testFormatPromptMessage(): void
+    {
+        $message = new PromptMessage(Role::User, new TextContent('hello'));
+        $result = (new PromptResultFormatter())->format($message);
+        $this->assertCount(1, $result);
+        $this->assertSame($message, $result[0]);
+    }
+
+    public function testFormatUserAssistantShorthand(): void
+    {
+        $result = (new PromptResultFormatter())->format([
+            'user' => 'Hello',
+            'assistant' => 'Hi there',
+        ]);
+        $this->assertCount(2, $result);
+        $this->assertSame(Role::User, $result[0]->role);
+        $this->assertSame(Role::Assistant, $result[1]->role);
+    }
+
+    public function testFormatRoleContentArray(): void
+    {
+        $result = (new PromptResultFormatter())->format([
+            ['role' => 'user', 'content' => 'Hello'],
+        ]);
+        $this->assertCount(1, $result);
+        $this->assertSame(Role::User, $result[0]->role);
+    }
+}