Fix hash collision in TypeMapLazyDictionary causing non-deterministic ArgumentException (#123502)

## Description

`TypeMapping.GetOrCreateExternalTypeMapping` threw non-deterministic
`ArgumentException` when type names hashed to the same value. For
example, `"Windows.Media.Devices.Core.FrameFlashControl"` and
`"Windows.Security.Credentials.PasswordCredential"` produce identical
`GetHashCode()` results, causing collision on insertion.

Root cause: `LazyExternalTypeDictionary` used `Dictionary<int,
DelayedType>` keyed by `string.GetHashCode()` instead of the string
itself.

This matches the collision-safe approach already used in
`LazyProxyTypeDictionary` but simplified since external type mappings
don't require multi-value support.

The fix addresses a design issue where hash collisions could occur. The
existing test coverage is sufficient to validate the behavior.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: AaronRobinsonMSFT <30635565+AaronRobinsonMSFT@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/TypeMapLazyDictionary.cs

@@ -317,14 +317,11 @@ public unsafe Type GetOrLoadType()
         [RequiresUnreferencedCode("Lazy TypeMap isn't supported for Trimmer scenarios")]
         private sealed class LazyExternalTypeDictionary : LazyTypeLoadDictionary<string>
         {
-            private static int ComputeHashCode(string key) => key.GetHashCode();
-
-            private readonly Dictionary<int, DelayedType> _lazyData = new();
+            private readonly Dictionary<string, DelayedType> _lazyData = new();
 
             protected override bool TryGetOrLoadType(string key, [NotNullWhen(true)] out Type? type)
             {
-                int hash = ComputeHashCode(key);
-                if (!_lazyData.TryGetValue(hash, out DelayedType? value))
+                if (!_lazyData.TryGetValue(key, out DelayedType? value))
                 {
                     type = null;
                     return false;
@@ -336,13 +333,12 @@ protected override bool TryGetOrLoadType(string key, [NotNullWhen(true)] out Typ
 
             public void Add(string key, TypeNameUtf8 targetType, RuntimeAssembly fallbackAssembly)
             {
-                int hash = ComputeHashCode(key);
-                if (_lazyData.ContainsKey(hash))
+                if (_lazyData.ContainsKey(key))
                 {
                     ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException(key);
                 }
 
-                _lazyData.Add(hash, new DelayedType(targetType, fallbackAssembly));
+                _lazyData.Add(key, new DelayedType(targetType, fallbackAssembly));
             }
         }