guard decide_and_execute without active run

[Rubayet19]

Description

Fail fast in Runtime.decide_and_execute when no run is active and add a regression test to ensure the executor never runs without a valid run context.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #145

Changes Made

• Added a guard in decide_and_execute to raise when there is no active run
• Added a test to verify no execution occurs without an active run

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (python3.11 -m pytest core/tests/test_runtime.py -v)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• 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