test sweep: cover WelcomeView open-project dialog (T-91) — crosses 90%

Four widget tests in test/builtin/welcome/dialog_test.dart driving
the WelcomeView open-folder fallback dialog. Uses a custom harness
that wraps the standard widget harness in a DialogHost so the
kernel.dialog.show() call inside _openFolder actually renders.

- MissingPluginException on pickDirectory → _OpenProjectDialog
  appears with the expected title / hint / Cancel / Open chrome.
- Cancel dismisses the modal cleanly.
- Open with an empty path is a no-op (early return in _submit).
- Open with a non-repo path keeps the dialog (project.open returns
  false, _loading flag flips back).

Coverage clears the **90% T-91 target**: 89.93% -> 90.53%. Floor
bumped to 90.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 89
+coverage_floor: 90
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads