K-Element Target Sum

You are given two integers k and n. Your task is to discover all valid combinations of exactly k distinct digits (from the set 1 through 9) that add up to exactly n.

Each digit in the set {1, 2, 3, 4, 5, 6, 7, 8, 9} may appear at most once in any combination. Additionally, every combination must be distinct—meaning no two combinations in the result should contain the exact same set of digits, regardless of their arrangement.

Return a list containing all such valid combinations. The combinations may be returned in any order, and within each combination, the digits may also appear in any order.

Key Rules:

1. Limited Digit Pool: You can only use digits from 1 to 9 (inclusive).
2. No Repetition Within a Combination: Each digit can be used at most once per combination.
3. Exact Count Requirement: Each combination must contain exactly k digits.
4. Target Sum: The sum of all digits in each combination must equal exactly n.
5. Uniqueness: The output should not contain duplicate combinations.