perf: replace OfType().Select().ToArray() with manual loop for dependency extraction

[thomhurst]

Two code paths build the Dependencies array via a LINQ chain that allocates an OfTypeIterator, a SelectArrayIterator, and the final array per test:

TUnit.Engine/Building/Collectors/AotTestDataCollector.cs:277:

Dependencies = result.Attributes.OfType<DependsOnAttribute>().Select(a => a.ToTestDependency()).ToArray(),

TUnit.Engine/Discovery/ReflectionTestDataCollector.cs:2213:

Dependencies = result.Attributes.OfType<DependsOnAttribute>().Select(static a => a.ToTestDependency()).ToArray(),

Manual loop with a known/upper-bound capacity:

List<TestDependency>? deps = null;
foreach (var attr in result.Attributes)
{
    if (attr is DependsOnAttribute dep)
    {
        (deps ??= new List<TestDependency>()).Add(dep.ToTestDependency());
    }
}
var dependencies = deps?.ToArray() ?? Array.Empty<TestDependency>();

For tests with zero DependsOnAttribute (the common case), this avoids both LINQ iterators and the empty-array allocation by returning the shared empty array.

Why hot: Per test built during discovery (both AOT and reflection paths).
TFM: No gating.