Guard against null workspaceFolders on initialize

[andyleejordan]

Fixes #2300.

Problem

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 the instance method GetFileSystemPath() on a null Uri threw. The initialize response was never returned and the server was unusable for that client. The same hazard exists everywhere else we dereference folder.Uri (WorkspacePaths, GetRelativePath, FindFileInWorkspace).

Investigation note

The issue describes this as Linux-only with a repro whose workspaceFolders entry has a valid uri. I drove a locally-built PSES over stdio and that exact payload does not throw. The real trigger is a workspace folder lacking a URI, which matches the captured stack trace pointing at PsesLanguageServer.cs:line 150 (the HostStartOptions initializer). Reproduced the NRE with a uriless folder and confirmed it's gone after this change.

Fix

• 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 / CWD behavior.
• Call it from OnInitialize instead of the inline null-check + AddRange.
• Add a regression test covering null input and uriless/null folders.

Validation

• dotnet build clean.
• Category=Workspace tests pass on net8.0 (7/7).
• Manually drove a built PSES over stdio with a uriless workspaceFolders entry: before, NRE in OnInitialize and no initialize response; after, the handshake completes normally.

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🤖 Drafted by Copilot (Claude Opus 4.8) — please review/edit before merging.

[Copilot]

Pull request overview

This PR fixes issue #2300 where a NullReferenceException is thrown in the OnInitialize handler when a client sends workspaceFolders containing an entry without a valid Uri. The fix centralizes workspace folder validation into a new method that filters out null folders and those lacking a URI before adding them to the service's collection.

Changes:

• Introduces WorkspaceService.AddWorkspaceFolders and a static GetValidWorkspaceFolders filter that guard against null/URI-less workspace folders.
• Replaces the inline null-check + AddRange call in PsesLanguageServer.OnInitialize with the new guarded AddWorkspaceFolders method.
• Adds regression tests covering null collections, null folder entries, and folders missing URIs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/PowerShellEditorServices/Services/Workspace/WorkspaceService.cs	Adds AddWorkspaceFolders (public) and GetValidWorkspaceFolders (internal static) to validate and filter workspace folders before adding them.
src/PowerShellEditorServices/Server/PsesLanguageServer.cs	Replaces the previous inline null-check + AddRange with a call to the new AddWorkspaceFolders method.
test/PowerShellEditorServices.Test/Session/WorkspaceTests.cs	Adds four unit tests covering the new filtering behavior for null inputs, empty collections, and folders without URIs.

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[andyleejordan]

@SeeminglyScience @JustinGrote bots solving bots' issues, but it's a real bug (from a user) and a real fix (a null check) with tests.

[JustinGrote]

I don't really have a way to test this without building a whole separate new extension, but the synthetic test passes on my side so any kind of mitigated null check is a good thing IMHO.