Add IM\ChatUser service for im.chat.user.* support

[mesilov]

Problem

The Bitrix24 REST API methods im.chat.user.* (chat participant management) are not yet supported by the SDK. See https://apidocs.bitrix24.com/api-reference/chats/index.html.

Proposed solution

Add a new service src/Services/IM/Chat/Service/ChatUser.php that wraps the following REST methods, following patterns in other src/Services/<Scope>/ folders. Scope: im.

Methods to implement

• im.chat.user.add — add participants to a chat
• im.chat.user.delete — remove a participant from a chat
• im.chat.user.list — list participant ids of a chat

Acceptance criteria

• Bitrix24\SDK\Services\IM\Chat\Service\ChatUser implements each method with correct parameter mapping and #[ApiEndpointMetadata] / #[ApiServiceMetadata(new Scope(['im']))] attributes
• IMServiceBuilder::chatUser() exposes the new service
• Unit tests in tests/Unit/Services/IM/Chat/Service/ChatUserTest.php pass (make test-unit)
• Integration tests in tests/Integration/Services/IM/Chat/Service/ChatUserTest.php pass
• make test-integration-im-chat-user target added to Makefile and a suite added to phpunit.xml.dist
• CHANGELOG.md entry under ## 3.2.0 – UNRELEASED → ### Added with link to this issue

Part of #422.