Guard against workspace folders without a URI on initialize

When a client sends `workspaceFolders` on `initialize` and one of the
entries has no `uri`, the `OnInitialize` handler threw a
`NullReferenceException` while resolving the initial working directory
(`workspaceService.WorkspaceFolders.FirstOrDefault()?.Uri.GetFileSystemPath()`).
The `?.` only guarded the folder being null, not its `Uri`, so calling
`GetFileSystemPath()` on a null `Uri` blew up, the `initialize` response
was never returned, and the server was unusable for that client. The same
hazard exists in every other place we dereference `folder.Uri`
(`WorkspacePaths`, `GetRelativePath`, `FindFileInWorkspace`).

The issue (#2300) reports this as Linux-only with a repro that includes a
valid `uri`; that exact payload doesn't actually throw (I reproduced both
ways by driving a built PSES over stdio). The real trigger is a workspace
folder lacking a URI, which is what the captured stack trace
(`PsesLanguageServer.cs:line 150`) points at.

- Add `WorkspaceService.AddWorkspaceFolders`, which owns the invariant that
  every folder in `WorkspaceFolders` has a non-null `Uri`. It skips null
  folders and folders without a URI (logging a warning) and treats a null
  collection as "no folders yet", falling back to the existing single-root
  and CWD behavior.
- Call it from `OnInitialize` instead of the inline null-check plus
  `AddRange`.
- Add a regression test covering null input and uriless/null folders.

Fixes #2300.

Drafted by Copilot (Claude Opus 4.8).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: andyleejordan

src/PowerShellEditorServices/Server/PsesLanguageServer.cs

@@ -140,10 +140,7 @@ public async Task StartAsync()
 
                             // Set the workspace path from the parameters.
                             WorkspaceService workspaceService = languageServer.Services.GetService<WorkspaceService>();
-                            if (initializeParams.WorkspaceFolders is not null)
-                            {
-                                workspaceService.WorkspaceFolders.AddRange(initializeParams.WorkspaceFolders);
-                            }
+                            workspaceService.AddWorkspaceFolders(initializeParams.WorkspaceFolders);
 
                             // Parse initialization options.
                             JObject initializationOptions = initializeParams.InitializationOptions as JObject;

src/PowerShellEditorServices/Services/Workspace/WorkspaceService.cs

@@ -106,6 +106,35 @@ public WorkspaceService(ILoggerFactory factory)
 
         public IEnumerable<string> WorkspacePaths => WorkspaceFolders.Select(i => i.Uri.GetFileSystemPath());
 
+        /// <summary>
+        /// Adds the given workspace folders, ignoring any that are null or lack a URI.
+        /// </summary>
+        /// <remarks>
+        /// Some LSP clients send workspace folders without a URI on initialize. Adding such a
+        /// folder would later throw a <see cref="NullReferenceException"/> when its URI is
+        /// dereferenced (e.g. when resolving the initial working directory or a relative path),
+        /// breaking the handshake. See
+        /// https://github.com/PowerShell/PowerShellEditorServices/issues/2300.
+        /// </remarks>
+        public void AddWorkspaceFolders(IEnumerable<WorkspaceFolder> workspaceFolders)
+        {
+            if (workspaceFolders is null)
+            {
+                return;
+            }
+
+            foreach (WorkspaceFolder workspaceFolder in workspaceFolders)
+            {
+                if (workspaceFolder?.Uri is null)
+                {
+                    logger.LogWarning("Ignored workspace folder without a URI: " + workspaceFolder?.Name);
+                    continue;
+                }
+
+                WorkspaceFolders.Add(workspaceFolder);
+            }
+        }
+
         /// <summary>
         /// Gets an open file in the workspace. If the file isn't open but exists on the filesystem, load and return it.
         /// <para>IMPORTANT: Not all documents have a backing file e.g. untitled: scheme documents.  Consider using

test/PowerShellEditorServices.Test/Session/WorkspaceTests.cs

@@ -86,6 +86,32 @@ internal static WorkspaceService FixturesWorkspace()
             };
         }
 
+        // Regression test for https://github.com/PowerShell/PowerShellEditorServices/issues/2300:
+        // a client that sends a workspace folder without a URI on initialize used to crash the
+        // server with a NullReferenceException when the URI was later dereferenced.
+        [Fact]
+        public void AddWorkspaceFoldersIgnoresNullAndUrilessFolders()
+        {
+            WorkspaceService workspace = new(NullLoggerFactory.Instance);
+
+            // Null collection is a no-op rather than a throw.
+            workspace.AddWorkspaceFolders(null);
+            Assert.Empty(workspace.WorkspaceFolders);
+
+            workspace.AddWorkspaceFolders(new[]
+            {
+                new WorkspaceFolder { Uri = DocumentUri.FromFileSystemPath(s_workspacePath), Name = "valid" },
+                new WorkspaceFolder { Name = "missing-uri" },
+                null
+            });
+
+            WorkspaceFolder folder = Assert.Single(workspace.WorkspaceFolders);
+            Assert.Equal("valid", folder.Name);
+
+            // The downstream dereferences that previously threw now succeed.
+            Assert.Equal(s_workspacePath, Assert.Single(workspace.WorkspacePaths));
+        }
+
         [Fact]
         public void HasDefaultForWorkspacePaths()
         {