chore(selenium): patch selenium in v3 pytest plugin for browser test tags (#16531)

## Description

The v3 pytest plugin (`ddtrace.testing.internal.pytest`) does not call `_patch_all()` by default, so the Selenium integration was never applied — browser tests were missing `test.is_browser`, `test.browser.*` tags.

This PR explicitly patches Selenium in `pytest_sessionstart` when the ddtrace trace filter is active (which is the default for the v3 plugin), ensuring browser test visibility tags are set even without `--ddtrace-patch-all`.

## Changes

- **`ddtrace/testing/internal/pytest/plugin.py`**: Patch Selenium in `pytest_sessionstart` when `enable_ddtrace_trace_filter` is true but `enable_all_ddtrace_integrations` is false.
- **`ddtrace/contrib/internal/selenium/patch.py`**: Added docstring explaining how the integration works with both v2 and v3 plugins.
- **`tests/contrib/selenium/test_selenium_chrome.py`**: Added `test_selenium_v3_plugin_tags` to verify browser tags are set with the v3 plugin. Existing snapshot tests are pinned to the old plugin (`DD_PYTEST_USE_NEW_PLUGIN=false`) since they rely on agent traces.

## Testing

- New test `test_selenium_v3_plugin_tags` uses `pytester` with a mocked `WebDriver` to verify `test.is_browser`, `test.browser.name`, and `test.browser.version` tags are correctly set.

## Risks

None — the Selenium patch is wrapped in a try/except and only applies in the v3 plugin path.

Co-authored-by: federico.mon <federico.mon@datadoghq.com>

Author: gnufede

ddtrace/testing/internal/pytest/plugin.py

@@ -320,6 +320,15 @@ def pytest_sessionstart(self, session: pytest.Session) -> None:
 
         if self.enable_all_ddtrace_integrations:
             enable_all_ddtrace_integrations()
+        elif self.enable_ddtrace_trace_filter:
+            # Patch Selenium so browser tests get test visibility tags (test.is_browser, test.browser.*).
+            # We create a root span with type=test in trace_context(); Selenium integration checks for that span.
+            try:
+                from ddtrace.contrib.internal.selenium.patch import patch as patch_selenium
+
+                patch_selenium()
+            except Exception:
+                log.debug("Could not patch Selenium for test visibility", exc_info=True)
 
     def pytest_sessionfinish(self, session: pytest.Session) -> None:
         # With xdist, the main process does not execute tests, so we cannot rely on the normal `session.get_status()`

tests/contrib/selenium/test_selenium_chrome.py

@@ -6,6 +6,7 @@
 """
 
 import http.server
+import json
 import multiprocessing
 import os
 from pathlib import Path
@@ -118,6 +119,7 @@ def test_selenium_local_pass():
                 CI_PROJECT_DIR=str(testdir.tmpdir),
                 DD_CIVISIBILITY_AGENTLESS_ENABLED="false",
                 _DD_CIVISIBILITY_DISABLE_EVP_PROXY="true",
+                # Snapshot test expects traces from the agent; v3 plugin uses TestOptWriter and does not send them.
                 DD_PYTEST_USE_NEW_PLUGIN="false",
             )
         ),
@@ -170,6 +172,7 @@ def test_selenium_local_pass():
                 CI_PROJECT_DIR=str(testdir.tmpdir),
                 DD_CIVISIBILITY_AGENTLESS_ENABLED="false",
                 _DD_CIVISIBILITY_DISABLE_EVP_PROXY="true",
+                # Snapshot test expects traces from the agent; v3 plugin uses TestOptWriter and does not send them.
                 DD_PYTEST_USE_NEW_PLUGIN="false",
             )
         ),
@@ -225,7 +228,70 @@ def test_selenium_local_unpatch():
                 CI_PROJECT_DIR=str(testdir.tmpdir),
                 DD_CIVISIBILITY_AGENTLESS_ENABLED="false",
                 _DD_CIVISIBILITY_DISABLE_EVP_PROXY="true",
+                # Snapshot test expects traces from the agent; v3 plugin uses TestOptWriter and does not send them.
                 DD_PYTEST_USE_NEW_PLUGIN="false",
             )
         ),
     )
+
+
+def test_selenium_v3_plugin_tags(tmp_path, pytester, git_repo):
+    events_file = tmp_path / "events.json"
+
+    # conftest.py that captures events emitted by the v3 plugin to a JSON file
+    pytester.makeconftest(
+        f"""
+import json, atexit
+from ddtrace.testing.internal.writer import TestOptWriter
+
+_events = []
+_orig_put = TestOptWriter.put_event
+def _capture(self, event):
+    _events.append(event)
+    return _orig_put(self, event)
+TestOptWriter.put_event = _capture
+
+@atexit.register
+def _dump():
+    with open(r"{events_file}", "w") as f:
+        json.dump(_events, f, default=str)
+"""
+    )
+
+    pytester.makepyfile(
+        test_selenium="""
+from selenium.webdriver.remote.webdriver import WebDriver
+
+def test_selenium_browser_tags():
+    driver = WebDriver.__new__(WebDriver)
+    driver.caps = {"browserName": "chrome", "browserVersion": "120.0"}
+    driver.add_cookie = lambda cookie: None
+    driver.execute = lambda *a, **kw: None
+
+    driver.get("http://example.com")
+"""
+    )
+
+    subprocess.run(
+        ["pytest", "--ddtrace", "-v", "-s"],
+        cwd=str(pytester.path),
+        env=_get_default_ci_env_vars(
+            dict(
+                DD_API_KEY="foobar.baz",
+                DD_CIVISIBILITY_ITR_ENABLED="false",
+                DD_PATCH_MODULES="sqlite3:false",
+                CI_PROJECT_DIR=str(pytester.path),
+                DD_CIVISIBILITY_AGENTLESS_ENABLED="false",
+                _DD_CIVISIBILITY_DISABLE_EVP_PROXY="true",
+                DD_PYTEST_USE_NEW_PLUGIN="true",
+            )
+        ),
+    )
+
+    events = json.loads(events_file.read_text())
+    test_events = [e for e in events if e["type"] == "test"]
+    meta = test_events[0]["content"]["meta"]
+    assert meta["test.is_browser"] == "true"
+    assert meta["test.browser.driver"] == "selenium"
+    assert meta["test.browser.name"] == "chrome"
+    assert meta["test.browser.version"] == "120.0"