Fix flaky Emacs e2e test: replace fixed sleep with diagnostic polling loop (#2276)

* Initial plan

* Fix timing issue in Emacs test by polling for flymake diagnostics

Agent-Logs-Url: https://github.com/PowerShell/PowerShellEditorServices/sessions/19f2e25d-3a5e-4570-97bf-ce36493b9795

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

---------

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

Author: Copilot

test/emacs-test.el

@@ -61,8 +61,12 @@
       (let ((lsp (eglot-current-server)))
         (should (string= (eglot--project-nickname lsp) "PowerShellEditorServices"))
         (should (member (cons 'powershell-mode "powershell") (eglot--languages lsp))))
-      (sleep-for 5) ; TODO: Wait for "textDocument/publishDiagnostics" instead
       (flymake-start)
+      ;; Wait for diagnostics to arrive instead of sleeping a fixed duration
+      (let ((deadline (time-add (current-time) 30)))
+        (while (and (time-less-p (current-time) deadline)
+                    (null (flymake-diagnostics)))
+          (sleep-for 0.5)))
       (goto-char (point-min))
       (flymake-goto-next-error)
       (should (eq 'flymake-warning (face-at-point))))))