Preserve caller-owned invite target streams (#758)

* Initial plan

* Handle user-provided invite streams

Co-authored-by: Lulalaby <[email protected]>

* Preserve caller-owned invite target streams

Co-authored-by: Lulalaby <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Lulalaby <[email protected]>

Author: Copilot

DisCatSharp/Net/Rest/DiscordApiClient.cs

@@ -3899,13 +3899,16 @@ internal async Task<DiscordInvite> CreateChannelInviteAsync(ulong channelId, int
 		var ret = DiscordJson.DeserializeObject<DiscordInvite>(res.Response, this.Discord);
 		ret.Discord = this.Discord;
 
-		try
-		{
-			targetUsersCsv?.Stream.Dispose();
-		}
-		catch
+		if (targetUsersCsv is not null && targetUsersCsv.ResetPositionTo is null)
 		{
-			// ignore
+			try
+			{
+				targetUsersCsv.Stream.Dispose();
+			}
+			catch
+			{
+				// ignore
+			}
 		}
 
 		return ret;
@@ -5152,13 +5155,16 @@ internal async Task UpdateInviteTargetUsersAsync(string inviteCode, Stream? targ
 
 		if (targetUsersCsv.ResetPositionTo is not null)
 			targetUsersCsv.Stream.Position = targetUsersCsv.ResetPositionTo.Value;
-		try
-		{
-			targetUsersCsv.Stream.Dispose();
-		}
-		catch
+		else if (targetUsersCsv is not null)
 		{
-			// ignore
+			try
+			{
+				targetUsersCsv.Stream.Dispose();
+			}
+			catch
+			{
+				// ignore
+			}
 		}
 	}