test: add integration tests for installer warning detection

- add make target test-integration and run it in reusable CI workflow
- add integration unittest covering _page_warnings stderr output + deduplication
- surface Matomo installer warnings during Playwright flow (stderr only)

https://chatgpt.com/share/694c1371-365c-800f-bdf8-ede2e850e648

tests/integration/test_web_installer_warnings.py

@@ -0,0 +1,126 @@
+import io
+import unittest
+from contextlib import redirect_stderr
+
+
+# Import the function under test.
+# This keeps the test close to real integration behavior without requiring Playwright.
+from matomo_bootstrap.installers.web import _page_warnings
+
+
+class _FakeLocatorNth:
+    def __init__(self, text: str):
+        self._text = text
+
+    def inner_text(self) -> str:
+        return self._text
+
+
+class _FakeLocator:
+    def __init__(self, texts: list[str]):
+        self._texts = texts
+
+    def count(self) -> int:
+        return len(self._texts)
+
+    def nth(self, i: int) -> _FakeLocatorNth:
+        return _FakeLocatorNth(self._texts[i])
+
+
+class _FakePage:
+    """
+    Minimal Playwright-like page stub:
+    - locator(selector) -> object with count() / nth(i).inner_text()
+    - url, title()
+    """
+
+    def __init__(self, *, url: str, title: str, selector_texts: dict[str, list[str]]):
+        self.url = url
+        self._title = title
+        self._selector_texts = selector_texts
+
+    def title(self) -> str:
+        return self._title
+
+    def locator(self, selector: str) -> _FakeLocator:
+        return _FakeLocator(self._selector_texts.get(selector, []))
+
+
+class TestWebInstallerWarningsIntegration(unittest.TestCase):
+    def test_detects_bootstrap_alert_warning_block(self) -> None:
+        """
+        Matomo installer commonly renders validation errors like:
+          <div class="alert alert-warning"> ... <ul><li>...</li></ul> ... </div>
+        We must detect and print those messages to stderr.
+        """
+        page = _FakePage(
+            url="http://matomo/index.php?action=setupSuperUser&module=Installation",
+            title="Superuser",
+            selector_texts={
+                # The key selector from the observed DOM
+                ".alert.alert-warning": [
+                    "Please fix the following errors:\n"
+                    "Password required\n"
+                    "Password (repeat) required\n"
+                    "The email doesn't have a valid format."
+                ],
+            },
+        )
+
+        buf = io.StringIO()
+        with redirect_stderr(buf):
+            warnings = _page_warnings(page, prefix="[install]")
+
+        # Function must return the warning text
+        self.assertEqual(len(warnings), 1)
+        self.assertIn("Please fix the following errors:", warnings[0])
+        self.assertIn("The email doesn't have a valid format.", warnings[0])
+
+        # And it must print it to stderr (stdout must remain token-only in the app)
+        out = buf.getvalue()
+        self.assertIn("[install] page warnings/errors detected", out)
+        self.assertIn("Superuser", out)
+        self.assertIn("The email doesn't have a valid format.", out)
+
+    def test_deduplicates_repeated_warning_blocks(self) -> None:
+        """
+        Some Matomo versions repeat the same alert in multiple containers.
+        We must return/log each unique text only once.
+        """
+        repeated = "Please fix the following errors:\nThe email doesn't have a valid format."
+        page = _FakePage(
+            url="http://matomo/index.php?action=setupSuperUser&module=Installation",
+            title="Superuser",
+            selector_texts={
+                ".alert.alert-warning": [repeated, repeated],
+            },
+        )
+
+        buf = io.StringIO()
+        with redirect_stderr(buf):
+            warnings = _page_warnings(page, prefix="[install]")
+
+        self.assertEqual(warnings, [repeated])
+
+        out = buf.getvalue()
+        # Only a single numbered entry should be printed
+        self.assertIn("[install]  1) ", out)
+        self.assertNotIn("[install]  2) ", out)
+
+    def test_no_output_when_no_warnings(self) -> None:
+        page = _FakePage(
+            url="http://matomo/",
+            title="Welcome",
+            selector_texts={},
+        )
+
+        buf = io.StringIO()
+        with redirect_stderr(buf):
+            warnings = _page_warnings(page, prefix="[install]")
+
+        self.assertEqual(warnings, [])
+        self.assertEqual(buf.getvalue(), "")
+
+
+if __name__ == "__main__":
+    unittest.main()