tests/workspace/builder(test[pane-readiness]): Add window_shell readiness skip coverage

why: The window_config.get("window_shell") fallback path at builder.py:566
was untested. Only pane-level shell: had fixture coverage.
what:
- Add skips_all_panes_with_window_shell fixture to PANE_READINESS_FIXTURES
- Verifies expected_wait_count=0 when window_shell is set

Author: tony

tests/workspace/test_builder.py

@@ -1587,6 +1587,20 @@ class PaneReadinessFixture(t.NamedTuple):
         ),
         expected_wait_count=1,
     ),
+    PaneReadinessFixture(
+        test_id="skips_all_panes_with_window_shell",
+        yaml=textwrap.dedent(
+            """\
+session_name: readiness-test
+windows:
+- window_shell: top
+  panes:
+  - shell_command: []
+  - shell_command: []
+""",
+        ),
+        expected_wait_count=0,
+    ),
 ]