Fix comparison in TaskHostParameters (#12733)

This change introduces a new read only struct that is used in Dictionary
for caching in TaskRegistry
#12620


Without a proper equality check it spawns a new process due to mismatch.

Experimental insertion without extra allocation reported:
https://devdiv.visualstudio.com/DevDiv/_git/VS/pullrequest/686770

Author: YuliiaKovalova

src/Build/Instance/TaskRegistry.cs

@@ -950,18 +950,16 @@ private static bool IdentityParametersMatch(in TaskHostParameters x, in TaskHost
                             return false;
                         }
 
-                        // For exact match, all properties must be equal (case-insensitive)
+                        // For exact match, only identity properties must be equal
+                        // Paths are NOT part of identity - they're just resolved locations
                         return string.Equals(x.Runtime, y.Runtime, StringComparison.OrdinalIgnoreCase) &&
                                string.Equals(x.Architecture, y.Architecture, StringComparison.OrdinalIgnoreCase) &&
-                               string.Equals(x.DotnetHostPath, y.DotnetHostPath, StringComparison.OrdinalIgnoreCase) &&
-                               string.Equals(x.MSBuildAssemblyPath, y.MSBuildAssemblyPath, StringComparison.OrdinalIgnoreCase) &&
                                x.TaskHostFactoryExplicitlyRequested == y.TaskHostFactoryExplicitlyRequested;
                     }
                     else
                     {
                         // Fuzzy match: null is treated as "don't care"
                         // Only check runtime and architecture for fuzzy matching
-
                         string runtimeX = x.Runtime;
                         string runtimeY = y.Runtime;
                         string architectureX = x.Architecture;

src/Framework/TaskHostParameters.cs

@@ -1,14 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 
 namespace Microsoft.Build.Framework
 {
     /// <summary>
     /// A readonly struct that represents task host parameters used to determine which host process to launch.
     /// </summary>
-    public readonly struct TaskHostParameters
+    public readonly struct TaskHostParameters : IEquatable<TaskHostParameters>
     {
         /// <summary>
         /// A static empty instance to avoid allocations when default parameters are needed.
@@ -72,6 +73,29 @@ internal TaskHostParameters(
         /// </summary>
         public bool? TaskHostFactoryExplicitlyRequested => _taskHostFactoryExplicitlyRequested;
 
+        public override bool Equals(object? obj) => obj is TaskHostParameters other && Equals(other);
+
+        public bool Equals(TaskHostParameters other) =>
+            StringComparer.OrdinalIgnoreCase.Equals(Runtime ?? string.Empty, other.Runtime ?? string.Empty)
+            && StringComparer.OrdinalIgnoreCase.Equals(Architecture ?? string.Empty, other.Architecture ?? string.Empty)
+            && TaskHostFactoryExplicitlyRequested == other.TaskHostFactoryExplicitlyRequested;
+
+        public override int GetHashCode()
+        {
+            // Manual hash code implementation for compatibility with .NET Framework 4.7.2
+            var comparer = StringComparer.OrdinalIgnoreCase;
+
+            unchecked
+            {
+                int hash = 17;
+                hash = hash * 31 + comparer.GetHashCode(Runtime ?? string.Empty);
+                hash = hash * 31 + comparer.GetHashCode(Architecture ?? string.Empty);
+                hash = hash * 31 + (TaskHostFactoryExplicitlyRequested?.GetHashCode() ?? 0);
+
+                return hash;
+            }
+        }
+
         /// <summary>
         /// Gets a value indicating whether returns true if parameters were unset.
         /// </summary>
@@ -134,7 +158,7 @@ internal TaskHostParameters WithTaskHostFactoryExplicitlyRequested(bool taskHost
         /// <summary>
         /// The method was added to sustain compatibility with ITaskFactory2 factoryIdentityParameters parameters dictionary.
         /// </summary>
-        internal Dictionary<string, string> ToDictionary() => new(3)
+        internal Dictionary<string, string> ToDictionary() => new(3, StringComparer.OrdinalIgnoreCase)
         {
             { nameof(Runtime), Runtime ?? string.Empty },
             { nameof(Architecture), Architecture ?? string.Empty },