test: skip generated extension bundle checks when missing

- Add a shared helper for reading the built extension background bundle.
- Skip the Firefox/WebExtension linter assertions when the generated bundle is absent.
- Keep fresh checkouts able to run pytest without building gitignored artifacts first.

tests/test_extension_error_page_handling.py

@@ -1,7 +1,15 @@
 from pathlib import Path
 
+import pytest
+
 ROOT = Path(__file__).resolve().parents[1]
 
+def read_built_background() -> str:
+    background_path = ROOT / "extension" / "background.js"
+    if not background_path.exists():
+        pytest.skip("extension/background.js is a generated build artifact; run npm run build:extension")
+    return background_path.read_text()
+
 def test_extension_retries_error_page_script_injection_before_failing():
     # core.ts was split into a core/ subfolder during the structure refactor:
     # the URL/error classifiers live in core/errors.ts and the executeScript
@@ -123,7 +131,7 @@ def test_tab_activation_open_and_merge_do_not_steal_audible_video_window():
     assert "const target = movableWindows.find(w => w.focused) || movableWindows[0];" in tabs
 
 def test_built_extension_avoids_static_firefox_unsupported_tab_group_api_refs():
-    background = (ROOT / "extension" / "background.js").read_text()
+    background = read_built_background()
 
     assert "chrome.tabGroups" not in background
     assert "chrome.tabs.group" not in background
@@ -132,7 +140,7 @@ def test_built_extension_avoids_static_firefox_unsupported_tab_group_api_refs():
     assert 'chrome.tabs["group"' in background
 
 def test_built_extension_avoids_direct_eval_token_for_firefox_linter():
-    background = (ROOT / "extension" / "background.js").read_text()
+    background = read_built_background()
 
     assert "(0, eval)(" not in background
     assert "eval(" not in background