Rewrite XML doc comments for consistency across the solution

[Copilot]

XML comments were inconsistent: abbreviated inline param/exception docs mixed with full expanded blocks, missing <typeparam> on generic types, empty <exception> tags, broken <see cref> and <paramref> references, a missing copyright header, and a typo.

Changes

• AbstractPolylineDecoder — Added <typeparam> docs; expanded the abbreviated Decode(polyline, CancellationToken) overload to full form including OperationCanceledException; added <param>/<exception> to ValidateFormat; added full docs to abstract GetReadOnlyMemory and CreateCoordinate; simplified private helper summaries to single sentences
• AbstractPolylineEncoder — Added <typeparam> docs; removed incorrect <exception cref="InternalBufferOverflowException"/> (wrong type); added description to empty <exception cref="InvalidOperationException"/>
• PolylineEncodingOptionsBuilder — Standardized all three builder <returns> to "The current PolylineEncodingOptionsBuilder instance for method chaining."; simplified WithPrecision summary
• PolylineEncoding — Removed stale <paramref name="rounding"/> (no such parameter on Normalize); fixed <see cref="GetCharCount"/> → <see cref="GetRequiredBufferSize"/>
• PolylineEncoderExtensions — Added missing copyright header; added <typeparam> docs; updated method summaries/returns to reference TCoordinate/TPolyline rather than concrete types; added <exception> for null coordinates
• PolylineDecoderExtensions — Added <exception cref="ArgumentNullException"> for null polyline on string and char[] overloads
• CoordinateDelta — Removed blank lines between closing doc tags and method declarations; moved second <summary> sentence to <remarks>
• Defaults — Added missing <summary> on Coordinate nested class; fixed typo "vakues" → "values"; added missing trailing period