Fix malformed query string in StreamInfo.ToUrl() causing 500 error via proxies

StreamInfo.ToUrl() generated URLs like `/master.m3u8?&DeviceId=...` (note `?&`)
because `?` was appended to the path and all parameters started with `&`. When
the first optional parameter (DeviceProfileId) was null, the result was a
malformed query string.

This is harmless when clients hit Jellyfin directly (ASP.NET Core tolerates `?&`),
but when accessed through a reverse proxy that parses and re-serializes the URL
(e.g. Home Assistant ingress via aiohttp/yarl), `?&` becomes `?=&` — introducing
an empty-key query parameter. ParseStreamOptions then crashes on `param.Key[0]`
with IndexOutOfRangeException.

Changes:
- StreamInfo.ToUrl(): Track query start position and replace the first `&` with
  `?` after all parameters are appended, producing valid query strings
- ParseStreamOptions: Guard against empty query parameter keys
- Tests: Remove .Replace("?&", "?") workaround that masked the bug

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

3 files changed, 12 insertions, 8 deletions

diff --git a/Jellyfin.Api/Helpers/StreamingHelpers.cs b/Jellyfin.Api/Helpers/StreamingHelpers.cs
index 1e984542e..c6823fa80 100644
--- a/Jellyfin.Api/Helpers/StreamingHelpers.cs
+++ b/Jellyfin.Api/Helpers/StreamingHelpers.cs
@@ -268,7 +268,7 @@ public static class StreamingHelpers
         Dictionary<string, string?> streamOptions = new Dictionary<string, string?>();
         foreach (var param in queryString)
         {
-            if (char.IsLower(param.Key[0]))
+            if (param.Key.Length > 0 && char.IsLower(param.Key[0]))
             {
                 // This was probably not parsed initially and should be a StreamOptions
                 // or the generated URL should correctly serialize it
diff --git a/MediaBrowser.Model/Dlna/StreamInfo.cs b/MediaBrowser.Model/Dlna/StreamInfo.cs
index 551bee89e..7aad97ce0 100644
--- a/MediaBrowser.Model/Dlna/StreamInfo.cs
+++ b/MediaBrowser.Model/Dlna/StreamInfo.cs
@@ -895,7 +895,7 @@ public class StreamInfo
 
         if (SubProtocol == MediaStreamProtocol.hls)
         {
-            sb.Append("/master.m3u8?");
+            sb.Append("/master.m3u8");
         }
         else
         {
@@ -906,10 +906,10 @@ public class StreamInfo
                 sb.Append('.');
                 sb.Append(Container);
             }
-
-            sb.Append('?');
         }
 
+        var queryStart = sb.Length;
+
         if (!string.IsNullOrEmpty(DeviceProfileId))
         {
             sb.Append("&DeviceProfileId=");
@@ -1133,6 +1133,12 @@ public class StreamInfo
             sb.Append(query);
         }
 
+        // Replace the first '&' with '?' to form a valid query string.
+        if (sb.Length > queryStart)
+        {
+            sb[queryStart] = '?';
+        }
+
         return sb.ToString();
     }
 
diff --git a/tests/Jellyfin.Model.Tests/Dlna/StreamInfoTests.cs b/tests/Jellyfin.Model.Tests/Dlna/StreamInfoTests.cs
index 8dea46806..4b3126fe1 100644
--- a/tests/Jellyfin.Model.Tests/Dlna/StreamInfoTests.cs
+++ b/tests/Jellyfin.Model.Tests/Dlna/StreamInfoTests.cs
@@ -216,8 +216,7 @@ public class StreamInfoTests
 
         string legacyUrl = streamInfo.ToUrl_Original(BaseUrl, "123");
 
-        // New version will return and & after the ? due to optional parameters.
-        string newUrl = streamInfo.ToUrl(BaseUrl, "123", null).Replace("?&", "?", StringComparison.OrdinalIgnoreCase);
+        string newUrl = streamInfo.ToUrl(BaseUrl, "123", null);
 
         Assert.Equal(legacyUrl, newUrl, ignoreCase: true);
     }
@@ -234,8 +233,7 @@ public class StreamInfoTests
             FillAllProperties(streamInfo);
             string legacyUrl = streamInfo.ToUrl_Original(BaseUrl, "123");
 
-            // New version will return and & after the ? due to optional parameters.
-            string newUrl = streamInfo.ToUrl(BaseUrl, "123", null).Replace("?&", "?", StringComparison.OrdinalIgnoreCase);
+            string newUrl = streamInfo.ToUrl(BaseUrl, "123", null);
 
             Assert.Equal(legacyUrl, newUrl, ignoreCase: true);
         }