Reduce allocation in random set index picking

[PaulusParssinen]

Casting returning type to Span<T> doesn't remove the backing heap allocation. We have to hand the responsibility of allocating the buffer to the caller. Extracted from #1100

We can sort get rid of these constants and calculate the threshold for the constant sized stackalloc on the fly with SpanOwner<T> like utility if this pattern gets more common in Garnet. I'm also using a bit conservative 1K threshold, could be higher if we wish so.

[Copilot]

Pull Request Overview

This PR refactors the random index picking logic in several object implementations by replacing calls that returned buffer allocations with an approach in which the caller supplies a buffer, thereby reducing heap allocations.

• Updated SortedSetObjectImpl.cs to conditionally use stackalloc and pass a buffer to RandomUtils.
• Modified SetObjectImpl.cs and HashObjectImpl.cs similarly to eliminate unneeded allocations.
• Updated RandomUtils.cs to accept a Span for filling random indexes instead of returning a new array.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
libs/server/Objects/SortedSet/SortedSetObjectImpl.cs	Replaced return logic with in-place buffer allocation using stackalloc.
libs/server/Objects/Set/SetObjectImpl.cs	Updated random index selection in both count positive and negative cases.
libs/server/Objects/Hash/HashObjectImpl.cs	Modified array length usage and in-place buffer population.
libs/common/RandomUtils.cs	Changed API to fill a provided Span and updated related helper logic.

Comments suppressed due to low confidence (2)

libs/server/Objects/Set/SetObjectImpl.cs:174

• The outer declaration of 'indexes' is shadowed by the inner declaration in each branch, which may cause confusion. Consider removing the outer declaration or reusing it to improve clarity.

var indexes = countParameter <= StackallocThreshold ? stackalloc int[StackallocThreshold].Slice(0, countParameter) : new int[countParameter];

libs/common/RandomUtils.cs:76

• The initialization loop that assigns 'shuffledIndexes[i] = i' is crucial for correct shuffling. Ensure that 'random.Shuffle(shuffledIndexes)' is well-tested to correctly randomize the array after this initialization.

for (var i = 0; i < shuffledIndexes.Length; i++)