Tokens can be cached beyond the lifetime of the (http) transport. (#834)

src/ModelContextProtocol.Core/Authentication/InMemoryTokenCache.cs

@@ -0,0 +1,27 @@
+
+namespace ModelContextProtocol.Authentication;
+
+/// <summary>
+/// Caches the token in-memory within this instance.
+/// </summary>
+internal class InMemoryTokenCache : ITokenCache
+{
+    private TokenContainer? _tokens;
+
+    /// <summary>
+    /// Cache the token.
+    /// </summary>
+    public ValueTask StoreTokensAsync(TokenContainer tokens, CancellationToken cancellationToken)
+    {
+        _tokens = tokens;
+        return default;
+    }
+
+    /// <summary>
+    /// Get the cached token.
+    /// </summary>
+    public ValueTask<TokenContainer?> GetTokensAsync(CancellationToken cancellationToken)
+    {
+        return new ValueTask<TokenContainer?>(_tokens);
+    }
+}