[Repo Assist] fix: handle None workspace.rootPath in FSI and Diagnostics commands (#2162)

* fix: handle None workspace.rootPath in FSI and Diagnostics commands

In multi-root workspaces, workspace.rootPath is None (undefined in
JavaScript), so using workspace.rootPath.Value throws at runtime.

This affects two code paths:
1. Fsi.fs sendCd fallback: when there is no active text editor and FSI
   needs a working directory, it would crash instead of falling back to
   a sensible directory.
2. Fsi.fs generateProjectReferencesForProject: calling
   'FSI: Generate project references' in a multi-root workspace would
   crash instead of creating the references.fsx document.
3. Diagnostics.fs writeToFile: running 'F#: Get diagnostic info' in a
   multi-root workspace would crash instead of opening the info file.

Fix: use workspace.rootPath first; fall back to the first workspace
folder if available; fall back to the OS temp directory as a last resort.

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

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

src/Components/Diagnostics.fs

@@ -110,7 +110,15 @@ Error: %A
 
     let writeToFile (text: string) =
         promise {
-            let path = node.path.join (workspace.rootPath.Value, "Diagnostic info")
+            let workspaceRoot =
+                workspace.rootPath
+                |> Option.defaultWith (fun () ->
+                    workspace.workspaceFolders
+                    |> Option.bind (fun folders ->
+                        if folders.Count > 0 then Some(string folders.[0].uri.fsPath) else None)
+                    |> Option.defaultValue (node.os.tmpdir ()))
+
+            let path = node.path.join (workspaceRoot, "Diagnostic info")
             let newFile = vscode.Uri.parse ("untitled:" + path)
             let! document = newFile |> workspace.openTextDocument
 

src/Components/Fsi.fs

@@ -331,7 +331,14 @@ module Fsi =
                 file, dir, line
 
             | None ->
-                let dir = workspace.rootPath.Value
+                let dir =
+                    workspace.rootPath
+                    |> Option.defaultWith (fun () ->
+                        workspace.workspaceFolders
+                        |> Option.bind (fun folders ->
+                            if folders.Count > 0 then Some(string folders.[0].uri.fsPath) else None)
+                        |> Option.defaultValue (node.os.tmpdir ()))
+
                 node.path.join (dir, "tmp.fsx"), dir, 1
 
         match lastCd with
@@ -666,7 +673,15 @@ module Fsi =
         let ctn = fsiInitCommandsForProject project
 
         promise {
-            let path = node.path.join (workspace.rootPath.Value, "references.fsx")
+            let workspaceRoot =
+                workspace.rootPath
+                |> Option.defaultWith (fun () ->
+                    workspace.workspaceFolders
+                    |> Option.bind (fun folders ->
+                        if folders.Count > 0 then Some(string folders.[0].uri.fsPath) else None)
+                    |> Option.defaultValue (node.os.tmpdir ()))
+
+            let path = node.path.join (workspaceRoot, "references.fsx")
 
             let! td = vscode.Uri.parse ("untitled:" + path) |> workspace.openTextDocument