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

src/Capability/Registry/ToolReference.php

@@ -11,8 +11,8 @@
 
 namespace Mcp\Capability\Registry;
 
+use Mcp\Capability\Formatter\ToolResultFormatter;
 use Mcp\Schema\Content\Content;
-use Mcp\Schema\Content\TextContent;
 use Mcp\Schema\Tool;
 
 /**
@@ -54,62 +54,7 @@ class ToolReference extends ElementReference
      */
     public function formatResult(mixed $toolExecutionResult): array
     {
-        if ($toolExecutionResult instanceof Content) {
-            return [$toolExecutionResult];
-        }
-
-        if (\is_array($toolExecutionResult)) {
-            if (empty($toolExecutionResult)) {
-                return [new TextContent('[]')];
-            }
-
-            $allAreContent = true;
-            $hasContent = false;
-
-            foreach ($toolExecutionResult as $item) {
-                if ($item instanceof Content) {
-                    $hasContent = true;
-                } else {
-                    $allAreContent = false;
-                }
-            }
-
-            if ($allAreContent && $hasContent) {
-                return $toolExecutionResult;
-            }
-
-            if ($hasContent) {
-                $result = [];
-                foreach ($toolExecutionResult as $item) {
-                    if ($item instanceof Content) {
-                        $result[] = $item;
-                    } else {
-                        $result = array_merge($result, $this->formatResult($item));
-                    }
-                }
-
-                return $result;
-            }
-        }
-
-        if (null === $toolExecutionResult) {
-            return [new TextContent('(null)')];
-        }
-
-        if (\is_bool($toolExecutionResult)) {
-            return [new TextContent($toolExecutionResult ? 'true' : 'false')];
-        }
-
-        if (\is_scalar($toolExecutionResult)) {
-            return [new TextContent($toolExecutionResult)];
-        }
-
-        $jsonResult = json_encode(
-            $toolExecutionResult,
-            \JSON_PRETTY_PRINT | \JSON_UNESCAPED_SLASHES | \JSON_UNESCAPED_UNICODE | \JSON_THROW_ON_ERROR | \JSON_INVALID_UTF8_SUBSTITUTE
-        );
-
-        return [new TextContent($jsonResult)];
+        return (new ToolResultFormatter())->format($toolExecutionResult);
     }
 
     /**