Get Project Chat

Get the chat for a specific project.

Args: project_id: The ID of the project to get chat for (validated for read access). agent_service: The AgentService instance (dependency). user_repo: User repository for resolving sender display names. project_repo: Project repository for checking project visibility. user_profile: Current user's profile for internal status check.

Returns: The chat history for the project, or None if no chat exists.

Raises: HTTPException: 400 for validation errors. HTTPException: 404 if project not found or user doesn't have access. HTTPException: 500 for unexpected errors.