perf: reduce ArrowBuffer builder reallocation work

[InCerryGit]

Summary

• Pre-reserve ArrowBuffer.Builder<T> capacity when AppendRange receives a known-count collection.
• Copy only the populated Length * sizeof(T) portion when reallocating builder storage.
• Add focused coverage for enumerable, read-only collection, and reallocation preservation paths.

The reallocation behavior preserves appended/current-length contents. Capacity beyond Length remains undefined and is not copied across reallocations.

Validation

• dotnet test test/Apache.Arrow.Tests/Apache.Arrow.Tests.csproj -c Release --filter "FullyQualifiedName~Apache.Arrow.Tests.ArrowBufferBuilderTests"
• rtk dotnet build "Apache.Arrow.sln" -c Release
• LSP diagnostics clean on changed files
• Code review completed before commit; no blockers found

[CurtHagenlocher]

Thanks!