Share page-check helpers in tests/page_check_common.py

MODES, TEST_FILL, FAKE_BRIDGE, the stub loader, the page opener and the no-AI case list now live in one place; sample_data and push move to tests/conftest.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/page_check_ai/test_ai_shared.py

@@ -2,18 +2,6 @@
 
 They use this folder's `page` and `push` fixtures, so they check the React page.
 """
-import sys
-from pathlib import Path
+from page_check_common import no_ai_cases
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "page_check"))
-
-from test_contract import (  # noqa: E402,F401
-    test_dark_toggle_flips_theme,
-    test_donut_and_legend_match_batch_states,
-    test_kpi_counts_match_batch_states,
-    test_log_lines_show_their_type,
-    test_no_bot_page_runs_on_sample_data,
-    test_plain_browser_first_open_shows_sample_data,
-    test_report_search_filters_rows_and_shows_bucket_pill,
-    test_step_list_follows_step_label,
-)
+globals().update(no_ai_cases())