Handling non-JWT token during OAuth by not caching

src/libraries/Client/Microsoft.Agents.Connector/RestClients/UserTokenRestClient.cs

@@ -422,14 +422,35 @@ private static void AddTokenResponseToCache(string cacheKey, TokenResponse token
         {
             if (tokenResponse != null && tokenResponse.Token != null)
             {
-                var jwtToken = new JwtSecurityToken(tokenResponse.Token);
-
-                tokenResponse.IsExchangeable = IsExchangeableToken(jwtToken);
-
                 if (tokenResponse.Expiration == null)
                 {
-                    // Token Service isn't returning Expiration in TokenResponse
-                    tokenResponse.Expiration = jwtToken.ValidTo;
+                    // Token Service isn't returning Expiration in TokenResponse.  Try to
+                    // get from token.
+
+                    if (tokenResponse.Token.StartsWith("gho_"))
+                    {
+                        // GitHub OAuth App docs indicate the tokens don't expire until revoked or unused
+                        // for one year.  For purposes of efficiency we'll cache it briefly.
+                        tokenResponse.Expiration = DateTimeOffset.UtcNow + TimeSpan.FromMinutes(30);
+                        tokenResponse.IsExchangeable = false;
+                    }
+                    else
+                    {
+                        try
+                        {
+                            var jwtToken = new JwtSecurityToken(tokenResponse.Token);
+                            tokenResponse.IsExchangeable = IsExchangeableToken(jwtToken);
+                            if (tokenResponse.Expiration == null)
+                            {
+                                tokenResponse.Expiration = jwtToken.ValidTo;
+                            }
+                        }
+                        catch (Exception)
+                        {
+                            // Since we don't have an expiration, we can't cache it.
+                            return;
+                        }
+                    }
                 }
 
                 _cache.Add(