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/ToolResultFormatterTest.php

@@ -0,0 +1,58 @@
+<?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\ToolResultFormatter;
+use Mcp\Schema\Content\TextContent;
+use PHPUnit\Framework\TestCase;
+
+class ToolResultFormatterTest extends TestCase
+{
+    public function testFormatStringResult(): void
+    {
+        $result = (new ToolResultFormatter())->format('hello');
+        $this->assertCount(1, $result);
+        $this->assertInstanceOf(TextContent::class, $result[0]);
+        $this->assertSame('hello', $result[0]->text);
+    }
+
+    public function testFormatContentResult(): void
+    {
+        $content = new TextContent('test');
+        $result = (new ToolResultFormatter())->format($content);
+        $this->assertSame([$content], $result);
+    }
+
+    public function testFormatArrayResult(): void
+    {
+        $result = (new ToolResultFormatter())->format(['key' => 'value']);
+        $this->assertCount(1, $result);
+        $this->assertInstanceOf(TextContent::class, $result[0]);
+        $this->assertStringContainsString('value', $result[0]->text);
+    }
+
+    public function testFormatNullResult(): void
+    {
+        $result = (new ToolResultFormatter())->format(null);
+        $this->assertCount(1, $result);
+        $this->assertInstanceOf(TextContent::class, $result[0]);
+        $this->assertSame('(null)', $result[0]->text);
+    }
+
+    public function testFormatBoolResult(): void
+    {
+        $result = (new ToolResultFormatter())->format(true);
+        $this->assertCount(1, $result);
+        $this->assertInstanceOf(TextContent::class, $result[0]);
+        $this->assertSame('true', $result[0]->text);
+    }
+}