Add PinchBench v2 task: sum off-by-one bug

[EloiRamos]

Task

Fix off-by-one error in sum function

Summary

This PR adds a new automated bug-fixing task for PinchBench v2.

The task requires fixing an off-by-one error in a function so that all tests pass.

Why this is useful

• Real-world debugging scenario
• Tests ability to interpret failing tests
• Requires minimal, precise fix
• Fully deterministic and automatically gradable

Included

• TASK_TEMPLATE-compliant task spec
• Self-contained workspace
• Deterministic grader

Scope

• Single task
• No external dependencies

[olearycrew]

@EloiRamos thanks for this - I'm a little confused. Is the idea that this is a task for Pinchbench? If so we need to give them a number - I think task 25 and 26 would work