Fix Resource::Contents#to_h to use correct property names per MCP spec

This PR fixes thr following fields with the MCP specification which requires:

- Text resources use `text` and `mimeType` fields
- Binary resources use `blob` (base64-encoded) and `mimeType` fields

https://modelcontextprotocol.io/specification/2025-11-25/server/resources#resource-contents

Fixes #234.

test/mcp/resource/contents_test.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Resource
+    class ContentsTest < ActiveSupport::TestCase
+      test "Contents#to_h returns hash with mimeType" do
+        contents = Resource::Contents.new(
+          uri: "test://example",
+          mime_type: "text/plain",
+        )
+
+        result = contents.to_h
+
+        assert_equal "test://example", result[:uri]
+        assert_equal "text/plain", result[:mimeType]
+        refute result.key?(:mime_type), "Should use camelCase 'mimeType' not snake_case 'mime_type'"
+      end
+
+      test "Contents#to_h omits mimeType when nil" do
+        contents = Resource::Contents.new(uri: "test://example")
+
+        result = contents.to_h
+
+        assert_equal({ uri: "test://example" }, result)
+        refute result.key?(:mimeType)
+      end
+
+      test "TextContents#to_h returns hash with text and mimeType" do
+        text_contents = Resource::TextContents.new(
+          uri: "test://text",
+          mime_type: "text/plain",
+          text: "Hello, world!",
+        )
+
+        result = text_contents.to_h
+
+        assert_equal "test://text", result[:uri]
+        assert_equal "text/plain", result[:mimeType]
+        assert_equal "Hello, world!", result[:text]
+        refute result.key?(:mime_type), "Should use camelCase 'mimeType' not snake_case 'mime_type'"
+      end
+
+      test "BlobContents#to_h returns hash with blob and mimeType" do
+        blob_contents = Resource::BlobContents.new(
+          uri: "test://binary",
+          mime_type: "image/png",
+          data: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+        )
+
+        result = blob_contents.to_h
+
+        assert_equal "test://binary", result[:uri]
+        assert_equal "image/png", result[:mimeType]
+        assert_equal "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==", result[:blob]
+        refute result.key?(:data), "Should use 'blob' not 'data' per MCP specification"
+        refute result.key?(:mime_type), "Should use camelCase 'mimeType' not snake_case 'mime_type'"
+      end
+
+      test "BlobContents#to_h omits mimeType when nil" do
+        blob_contents = Resource::BlobContents.new(
+          uri: "test://binary",
+          mime_type: nil,
+          data: "base64data",
+        )
+
+        result = blob_contents.to_h
+
+        assert_equal({ uri: "test://binary", blob: "base64data" }, result)
+        refute result.key?(:mimeType)
+      end
+    end
+  end
+end