Add pending task tracking to Guest for workload monitoring

[skycastlelily]

   Add centralized guest release tracking and skip operations on released guests

  - Add is_released property to Guest class to safely check if guest was released early
  - Implement thread-safe guest release tracking in Provision step with _released_guests set
  - Skip push/pull operations on released guests to prevent errors
  - Skip cleanup and finish operations on released guests
  - Add @container decorators to PushTask and PullTask classes

Pull Request Checklist

• implement the feature
• write the documentation
• extend the test coverage
• update the specification
• adjust plugin docstring
• modify the json schema
• mention the version
• include a release note

[gemini-code-assist]

Code Review

This pull request introduces pending task tracking on Guest objects, which is a valuable addition for workload monitoring and resource management. The implementation is thread-safe and handles task increments and decrements correctly, including in abort scenarios.

I've identified two areas for improvement in tmt/steps/execute/__init__.py to enhance code clarity and conciseness. One is simplifying a loop for finding a discover phase, and the other is removing a redundant condition check.