Add xfailed pretty test for C# 13 params collections call sites

The fixture is the desired output for params-collection call sites the
decompiler does not reconstruct yet, Assert.Ignore'd pending #829. All
four Roslyn 4.14/latest debug/opt configs compile the fixture and fail
only at the output comparison. Defects the fixture pins down:

- params ReadOnlySpan<T> call sites with non-constant or string
  elements leak the synthesized inline-array structs (declared with
  unspeakable escaped names like _003C_003Ey__InlineArray2<T>), and an
  empty span call prints default(ReadOnlySpan<int>); affects method,
  constructor, and indexer call sites alike.
- params IEnumerable<T>/IReadOnlyList<T> call sites (including
  delegate invocations) leak the compiler-synthesized
  <>z__ReadOnlyArray/<>z__ReadOnlySingleElementList wrapper classes.
- params List<T>/IList<T>/ICollection<T> call sites keep the raw
  CollectionsMarshal.SetCount/AsSpan lowering; params ImmutableArray<T>
  keeps ImmutableCollectionsMarshal.AsImmutableArray.
- params arguments are never collapsed in attribute arguments.
- local-function call sites get no params folding, and the params
  modifier is dropped from local-function declarations (a span
  parameter prints as scoped ReadOnlySpan<T> instead).
- natural-typed lambdas with params parameters print unspeakable
  <>f__AnonymousDelegate types and lose the params modifier.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -433,6 +433,7 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 				"mscorlib.dll",
 				"System.dll",
 				"System.Collections.dll",
+				"System.Collections.Immutable.dll",
 				"System.Console.dll",
 				"System.Core.dll",
 				"System.Linq.dll",

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -788,6 +788,13 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task ParamsCollectionsCallSites([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			Assert.Ignore("Params collections call sites are not yet reconstructed by the decompiler. See https://github.com/icsharpcode/ILSpy/issues/829");
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task RefStructInterfaces([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ParamsCollectionsCallSites.cs

@@ -0,0 +1,114 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	public class ParamsCollectionsCallSites
+	{
+		public delegate void ParamsEnumerableDelegate(params IEnumerable<int> values);
+
+		public class ParamsAttribute : Attribute
+		{
+			public ParamsAttribute(params int[] values)
+			{
+			}
+		}
+
+		public int this[params ReadOnlySpan<int> values] => values.Length;
+
+		public ParamsCollectionsCallSites(params ReadOnlySpan<string> values)
+		{
+		}
+
+		public static void ParamsEnumerable(params IEnumerable<int> values)
+		{
+		}
+		public static void ParamsGenericEnumerable<T>(params IEnumerable<T> values)
+		{
+		}
+		public static void ParamsGenericSpan<T>(params ReadOnlySpan<T> values)
+		{
+		}
+		public static void ParamsICollection(params ICollection<int> values)
+		{
+		}
+		public static void ParamsIList(params IList<int> values)
+		{
+		}
+		public static void ParamsIReadOnlyList(params IReadOnlyList<int> values)
+		{
+		}
+		public static void ParamsImmutableArray(params ImmutableArray<int> values)
+		{
+		}
+		public static void ParamsList(params List<int> values)
+		{
+		}
+		public static void ParamsReadOnlySpan(params ReadOnlySpan<int> values)
+		{
+		}
+		public static void ParamsSpanOfString(params ReadOnlySpan<string> values)
+		{
+		}
+		public static void SpanCallSites(int x)
+		{
+			ParamsReadOnlySpan();
+			ParamsReadOnlySpan(x);
+			ParamsReadOnlySpan(x, x + 1);
+			ParamsSpanOfString("a", "b");
+			ParamsGenericSpan("a", "b");
+		}
+		public static void EnumerableCallSites(int x)
+		{
+			ParamsEnumerable(1);
+			ParamsEnumerable(1, 2, 3);
+			ParamsEnumerable(x, x + 1);
+			ParamsIReadOnlyList(1, 2);
+			ParamsGenericEnumerable(1.0, 2.0);
+		}
+		public static void ListCallSites(int x)
+		{
+			ParamsList();
+			ParamsList(1, 2, 3);
+			ParamsIList(1, 2);
+			ParamsICollection(x);
+		}
+		public static void ImmutableArrayCallSites(int x)
+		{
+			ParamsImmutableArray();
+			ParamsImmutableArray(1, 2, 3);
+			ParamsImmutableArray(x, x + 1);
+		}
+		public static int MemberCallSites(int x)
+		{
+			return new ParamsCollectionsCallSites("a", "b")[1, 2, x];
+		}
+		public static void DelegateCallSites(ParamsEnumerableDelegate d)
+		{
+			d(4, 5);
+		}
+		[Params(1, 2, 3)]
+		public static void AttributeCallSite()
+		{
+		}
+		public static void LocalFunctionCallSites()
+		{
+			LocalArray();
+			LocalArray(1, 2, 3);
+			LocalSpan(1, 2);
+			static void LocalArray(params int[] values)
+			{
+			}
+			static void LocalSpan(params ReadOnlySpan<int> values)
+			{
+			}
+		}
+		public static int NaturalTypeLambdas()
+		{
+			var f = (params ReadOnlySpan<int> xs) => xs.Length;
+			var g = (params IEnumerable<int> xs) => 0;
+			return f(1, 2) + g(3);
+		}
+	}
+}