PR: Add 'is_running' attribute to TaskManagerBase and minor improvements to code

[jnsebgosselin]

This pull request adds an is_running property to the TaskManagerBase class, providing a clear and reliable way to check whether the task manager is currently processing tasks or has an active worker thread. Additionally, several minor code improvements and refactorings are included for better readability and maintainability.

Changes

• Add is_running attribute:
  • Implements a property that returns True when tasks are pending, running, or when the worker thread is active.
  • Improves thread state management and makes it easier for client code to monitor task execution status.
• Minor code refactoring:
  • Adds missing initializations for task dictionaries.
  • Cleans up comments and docstrings for clarity.
  • Enhances code style and consistency.