refactor: remove deprecated OAuth authentication base classes (#9337)

* refactor: remove deprecated OAuth authentication base classes

Remove abstract/generic auth infrastructure superseded by the concrete
implementations introduced in #9330 (LeanOAuthTokenHandler, LeanTokenHandler,
LeanTokenCredentials):

- Delete OAuthTokenHandler<TRequest, TResponse>
- Delete abstract TokenHandler<T> base
- Delete TokenCredentials (replaced by LeanTokenCredentials)
- Delete abstract AccessTokenMetaDataRequest/Response base classes
- Update XML documentation in LeanOAuthTokenHandler and LeanTokenHandler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: remove racy lockless fast path from LeanOAuthTokenHandler

* fix: remove redundant AccessToken empty check on token refresh

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Romazes

Brokerages/Authentication/AccessTokenMetaDataRequest.cs

@@ -25,6 +25,15 @@ namespace QuantConnect.Brokerages.Authentication
     /// </summary>
     public class LeanOAuthTokenHandler : LeanOAuthTokenHandler<LeanTokenCredentials>
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="LeanOAuthTokenHandler"/> class with default token credentials type.
+        /// </summary>
+        /// <param name="apiClient">The API client used to communicate with the Lean platform.</param>
+        /// <param name="request">The request model used to generate the access token.</param>
+        /// <param name="tokenLifetime">
+        /// The expected lifetime of a fetched token. A 1-minute safety buffer is applied before expiry.
+        /// Must be provided explicitly — each brokerage has a different token lifetime.
+        /// </param>
         public LeanOAuthTokenHandler(ApiConnection apiClient, OAuthTokenRequest request, TimeSpan tokenLifetime)
             : base(apiClient, request, tokenLifetime)
         {
@@ -71,14 +80,13 @@ public class LeanOAuthTokenHandler<T> : LeanTokenHandler<T>
 
         /// <summary>
         /// Stores the current access token and its type used for authenticating requests to the Lean platform.
-        /// Written inside <see cref="_lock"/>; read outside the lock via volatile fast path.
+        /// Always accessed inside <see cref="_lock"/>.
         /// </summary>
         private T _tokenCredentials;
 
         /// <summary>
         /// The UTC timestamp after which the cached token should be refreshed.
-        /// Always written inside <see cref="_lock"/> before <see cref="_tokenCredentials"/> is set,
-        /// so that a volatile read of <see cref="_tokenCredentials"/> guarantees visibility of this field.
+        /// Always accessed inside <see cref="_lock"/>.
         /// </summary>
         private DateTime _tokenExpiresAt;
 
@@ -106,21 +114,14 @@ public LeanOAuthTokenHandler(ApiConnection apiClient, OAuthTokenRequest request,
         /// <summary>
         /// Retrieves a valid access token from the Lean platform.
         /// Caches and reuses tokens until expiration to minimize unnecessary requests.
-        /// Retries up to <see cref="MaxRetryCount"/> times on failure, and is thread-safe via double-checked locking.
+        /// Retries up to <see cref="MaxRetryCount"/> times on failure. Thread-safe via a lock.
         /// </summary>
         /// <param name="cancellationToken">A token used to observe cancellation requests.</param>
-        /// <returns>A <see cref="TokenCredentials"/> containing the token type and access token string.</returns>
+        /// <returns>A <see cref="LeanTokenCredentials"/> instance containing the token type and access token string.</returns>
         public override T GetAccessToken(CancellationToken cancellationToken)
         {
-            // Fast path: return cached token without acquiring the lock
-            if (_tokenCredentials != null && DateTime.UtcNow < _tokenExpiresAt)
-            {
-                return _tokenCredentials;
-            }
-
             lock (_lock)
             {
-                // Second check: another thread may have refreshed while we waited for the lock
                 if (_tokenCredentials != null && DateTime.UtcNow < _tokenExpiresAt)
                 {
                     return _tokenCredentials;
@@ -136,8 +137,6 @@ public override T GetAccessToken(CancellationToken cancellationToken)
                         {
                             if (response.Success)
                             {
-                                // Write expiry before credentials — the volatile write of _tokenCredentials
-                                // acts as a release fence, ensuring the fast-path reader sees _tokenExpiresAt.
                                 _tokenExpiresAt = DateTime.UtcNow + _tokenLifetime - OffsetBeforeExpiration;
                                 return _tokenCredentials = response;
                             }

Brokerages/Authentication/AccessTokenMetaDataResponse.cs

@@ -41,12 +41,13 @@ public abstract class LeanTokenHandler<T> : DelegatingHandler
         private readonly Func<TokenType, string, AuthenticationHeaderValue> _createAuthHeader;
 
         /// <summary>
-        /// Initializes a new instance of the <see cref="LeanTokenHandler"/> class.
+        /// Initializes a new instance of the <see cref="LeanTokenHandler{T}"/> class.
         /// </summary>
         /// <param name="createAuthHeader">
         /// An optional delegate for creating an <see cref="AuthenticationHeaderValue"/>
         /// from the token type and access token. If not provided, a default implementation is used.
         /// </param>
+        /// <param name="handler">An optional inner <see cref="HttpMessageHandler"/>. If not provided, a default <see cref="HttpClientHandler"/> is used.</param>
         protected LeanTokenHandler(Func<TokenType, string, AuthenticationHeaderValue> createAuthHeader = null, HttpClientHandler handler = null)
             : base(handler ?? new HttpClientHandler())
         {
@@ -59,9 +60,7 @@ protected LeanTokenHandler(Func<TokenType, string, AuthenticationHeaderValue> cr
         /// with optional support for caching and refresh logic.
         /// </summary>
         /// <param name="cancellationToken">A cancellation token that can be used to cancel the token retrieval operation.</param>
-        /// <returns>
-        /// A <see cref="TokenCredentials"/> instance containing the token type and access token string.
-        /// </returns>
+        /// <returns>A <see cref="LeanTokenCredentials"/> instance containing the token type and access token string.</returns>
         public abstract T GetAccessToken(CancellationToken cancellationToken);
 
         /// <summary>