🪲 BUG-#205: Fix PR #202 review issues — duration, executor leak, checkpoint, docstring

[FernandoCelmer]

Description

Fixes all issues raised in PR #202 code review.

Issue: 📌 ISSUE-#205

Changes

• Duration in finally — task.duration now calculated in finally block, always available for tracer (was only set on success)
• Executor leak — _execute_with_timeout uses finally for executor.shutdown() (was missing on success path)
• Class error detection — _is_class_internal_error checks ExecutionWithClassError type (was only checking message patterns)
• Single storage query — _has_checkpoint + _restore_checkpoint unified into _try_restore_checkpoint (was 2 queries per checkpoint)
• Action docstring — Updated to reflect that retry/timeout are enforced by TaskEngine, not Action

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes