fix: make CancellationToken optional in AbstractPolylineEncoder.Encode

Avoids breaking existing callers that use the concrete class directly.
Only one Encode overload exists so there is no RS0026 concern.

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/0ad5e8c4-b82b-4f8c-805d-29afa8c45722

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

src/PolylineAlgorithm/Abstraction/AbstractPolylineEncoder.cs

@@ -74,7 +74,7 @@ protected AbstractPolylineEncoder(PolylineEncodingOptions options) {
     /// Thrown when the internal encoding buffer cannot accommodate the encoded value.
     /// </exception>
     [System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "MA0051:Method is too long", Justification = "Method contains local methods. Actual method only 55 lines.")]
-    public TPolyline Encode(ReadOnlySpan<TCoordinate> coordinates, CancellationToken cancellationToken) {
+    public TPolyline Encode(ReadOnlySpan<TCoordinate> coordinates, CancellationToken cancellationToken = default) {
         const string OperationName = nameof(Encode);
 
         _logger