Fix null comparison semantics, restore API backward compat, fix PackageIconUrl

[Copilot]

Addresses three issues flagged in review of the net10.0 support PR.

Changes

• CompareTo null ordering (PackageEntry.cs, CatalogPageEntry.cs): Both methods returned -1 for a null other, meaning "this < null" — backwards from the conventional "null sorts before non-null." Changed to return 1. The relational operators derived from CompareTo now behave correctly:

  null < nonNull  → true   ✓
  nonNull < null  → false  ✓
  nonNull > null  → true   ✓
  

• API backward compatibility (ProcessEntriesUtility.cs): Moving CancellationToken to last position (CA1068) broke existing callers using the original (token, entries) order. New CA1068-compliant signatures are the primary API; original-order overloads are preserved as [Obsolete] with #pragma warning disable CA1068:

  // Primary (new)
  public static Task<IReadOnlyList<FileInfo>> DownloadNuspecsAsync(..., IEnumerable<CatalogEntry> entries, CancellationToken token)
  
  // Backward compat
  [Obsolete("Use the overload with CancellationToken as the last parameter.")]
  public static Task<IReadOnlyList<FileInfo>> DownloadNuspecsAsync(..., CancellationToken token, IEnumerable<CatalogEntry> entries)

• PackageIconUrl formatting (build/config.props): URL was split across lines with leading whitespace, which MSBuild preserves verbatim, producing a malformed URL in NuGet metadata. Collapsed to a single line.

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