test(foundation): add overflow tests for Backoff

[SH20RAJ]

Adds unit tests to verify that Backoff::Linear and Backoff::Exponential handle overflow correctly using saturating arithmetic. The tests cover: 1) Linear backoff saturation when initial + increment * attempt overflows, and 2) Exponential backoff saturation and max cap when exponent is huge. This ensures the fix for integer overflow in Backoff::Linear remains effective.