test: make extension packaging tests build-independent

- Use a temporary fake extension tree in packaging tests instead of relying on ignored generated bundles.

- Keep coverage for Web Store manifest.key stripping and local-package key retention.

- Fix plain pytest runs in clean CI checkouts where extension/background.js has not been built yet.

tests/test_extension_packaging.py

@@ -11,8 +11,30 @@ def _load_packager():
     spec.loader.exec_module(module)
     return module
 
-def test_webstore_package_strips_manifest_key(tmp_path):
+def _fake_extension(tmp_path: Path) -> Path:
+    extension = tmp_path / "extension"
+    icons = extension / "icons"
+    icons.mkdir(parents=True)
+    (extension / "manifest.json").write_text(json.dumps({
+        "manifest_version": 3,
+        "name": "browser-cli",
+        "version": "1.2.3",
+        "key": "test-key",
+    }), encoding="utf-8")
+    for name in ("background.js", "content-dispatch.js", "content.js"):
+        (extension / name).write_text("// generated test bundle\n", encoding="utf-8")
+    (extension / "icon.svg").write_text("<svg />\n", encoding="utf-8")
+    (icons / "icon-128.png").write_bytes(b"png")
+    return extension
+
+def _packager_with_fake_extension(tmp_path: Path):
     packager = _load_packager()
+    packager.EXTENSION_DIR = _fake_extension(tmp_path)
+    packager.DIST_DIR = tmp_path / "dist"
+    return packager
+
+def test_webstore_package_strips_manifest_key(tmp_path):
+    packager = _packager_with_fake_extension(tmp_path)
     out = packager.package_extension(webstore=True, out=tmp_path / "webstore.zip")
 
     with zipfile.ZipFile(out) as zf:
@@ -26,10 +48,10 @@ def test_webstore_package_strips_manifest_key(tmp_path):
     assert "icons/icon-128.png" in names
 
 def test_local_package_keeps_manifest_key(tmp_path):
-    packager = _load_packager()
+    packager = _packager_with_fake_extension(tmp_path)
     out = packager.package_extension(webstore=False, out=tmp_path / "local.zip")
 
     with zipfile.ZipFile(out) as zf:
         manifest = json.loads(zf.read("manifest.json"))
 
-    assert "key" in manifest
+    assert manifest["key"] == "test-key"