foundations/09_sumAll : Clarify negative input handling

[shauryaa108]

Because

The Exercise 09 (sumAll) README states that the function accepts positive integers, but it does not explicitly explain how negative inputs should be handled. The tests expect negative inputs to result in an error, which may be confusing for learners.

This PR

• Adds a note clarifying that negative inputs should return an error.
• Improves the exercise instructions to better match the expected behavior tested by the exercise.

Issue

Closes #652

Additional Information

N/A

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the required format
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• This PR addresses an open issue and is linked in the Issue section
• This PR does not affect the exercise solution, so no solution updates were required