Add ArrayBuilder.AddRange and use it instead of List.ToArray pattern

[Copilot]

• Add AddRange method to ArrayBuilder<T> in /src/libraries/Common/src/System/Collections/Generic/ArrayBuilder.cs
• Refactor Directory.cs to use ArrayBuilder<T> with AddRange instead of new List<T>(enumerable).ToArray()
• Refactor DirectoryInfo.cs to use ArrayBuilder<T> with AddRange instead of new List<T>(enumerable).ToArray()
• Refactor Timer.cs to use ArrayBuilder<T> with AddRange instead of new List<T>(enumerable).ToArray()
• Add tests for ArrayBuilder<T>.AddRange method
• Optimize AddRange to pre-allocate for ICollection<T> inputs
• Add AddRange method to NativeAOT's ArrayBuilder<T> in /src/coreclr/tools/Common/System/Collections/Generic/ArrayBuilder.cs
• Add tests specifically for ICollection<T> optimization:
  • AddRange_ICollection_PreallocatesCapacity - verifies capacity is pre-allocated for ICollection inputs
  • AddRange_ICollection_EmptyCollection - verifies empty ICollection works correctly
  • AddRange_ICollection_AfterExistingItems - verifies ICollection optimization works when items already exist
  • AddRange_NonICollection_GrowsIncrementally - verifies non-ICollection path grows incrementally
• Build and test to verify changes work correctly

Original prompt

There are a few locations in src/libraries/System.Private.CoreLib/src/System/IO/Directory.cs src/System/IO/DirectoryInfo.cs, and src/libraries/System.Private.CoreLib/src/System/Threading/Timer.cs that have code of the pattern new List<concretetype>(enumerableData).ToArray(). They do this because they're in System.Private.Corelib, and don't have access to LINQ's ToArray method. However, these locations can be improved by using System.Collections.Generic.ArrayBuilder<T>, which is in corelib and can potentially avoid intermediate allocations. Refactor these locations to use ArrayBuilder<T>, and do it by adding an AddRange helper to ArrayBuilder to encapsulate the enumeration loop and keep callsites fairly simple. You don't need to search for any instances of this code pattern outside the files I've listed; they exist, but they're all tests and don't really need to be adjusted now.

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[Copilot]

Pull request overview

This PR adds an AddRange method to ArrayBuilder<T> to improve performance in locations that previously used the new List<T>(enumerable).ToArray() pattern. The implementation optimizes for ICollection<T> inputs by pre-allocating capacity, while falling back to incremental growth for general IEnumerable<T> inputs.

Key changes:

• Added AddRange(IEnumerable<T>) method to both Common and NativeAOT ArrayBuilder<T> implementations with ICollection<T> optimization
• Refactored I/O methods in Directory.cs, DirectoryInfo.cs, and Timer.cs to use ArrayBuilder<T>.AddRange() instead of List<T>.ToArray()
• Added comprehensive test coverage including tests for the ICollection optimization path, empty collections, and incremental growth

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/Common/src/System/Collections/Generic/ArrayBuilder.cs	Adds AddRange method with ICollection optimization for pre-allocating capacity
src/coreclr/tools/Common/System/Collections/Generic/ArrayBuilder.cs	Adds AddRange method to NativeAOT's ArrayBuilder with same optimization
src/libraries/System.Private.CoreLib/src/System/IO/Directory.cs	Refactors three methods (GetFiles, GetDirectories, GetFileSystemEntries) to use ArrayBuilder.AddRange instead of List.ToArray pattern
src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs	Refactors three methods (GetFiles, GetFileSystemInfos, GetDirectories) to use ArrayBuilder.AddRange instead of List.ToArray pattern
src/libraries/System.Private.CoreLib/src/System/Threading/Timer.cs	Refactors debugger proxy property to use ArrayBuilder.AddRange instead of List.ToArray pattern
src/libraries/Common/tests/Tests/System/Collections/Generic/ArrayBuilderTests.cs	Adds comprehensive test coverage for AddRange including ICollection optimization, empty collections, and non-ICollection paths