fix(2): F2-3 systemic — harness.browser.goto_with_retry; applied to all install overlays

Phase 2 lesson from F2-3 (n8n install Playwright flake on net::ERR_NETWORK_CHANGED): every
install overlay that does page.goto needs the same try/except PlaywrightError + status retry.
Centralize in runner/harness/browser.py::goto_with_retry; apply to ALL install overlays.

- runner/harness/browser.py: shared helper. Polls page.goto until status in accept_statuses;
  catches PlaywrightError (net::ERR_*) as a retryable signal, not a failure. Raises AssertionError
  with last_status + last_err diagnostic only on deadline expiry.
- tests/custom-html/test_install.py: now uses goto_with_retry (200 only, wait_until=load).
- tests/custom-html/playwright/test_browser_smoke.py: same.
- tests/n8n/test_install.py: replaced inline retry loop with goto_with_retry (200, 304).
- tests/keycloak/test_install.py: goto_with_retry for admin console (200, 302, 303; 45s goto).
- tests/cryptpad/test_install.py: goto_with_retry (200, 304; 60s goto, wait_until=load).
- tests/lasuite-docs/test_install.py: goto_with_retry (200, 301, 302; 60s goto).

Cold-verifiable: ssh cc-ci 'RECIPE=custom-html cc-ci-run runner/run_recipe_ci.py'
  all 5 stages PASS (including the install overlay that flaked in the deps_smoke run),
  deploy-count=1, head_ref=8a026066==chaos-version=8a026066 (HC1 non-vacuous).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/n8n/test_install.py

@@ -8,7 +8,7 @@ import os
 import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
-from harness import generic, lifecycle  # noqa: E402
+from harness import browser as harness_browser, generic, lifecycle  # noqa: E402
 
 
 def test_serving_and_editor(live_app, meta):
@@ -21,9 +21,9 @@ def test_serving_and_editor(live_app, meta):
 
     # A real browser loads the live n8n editor SPA over HTTPS.
     # n8n's boot is staged: /healthz returns 200 before / route is registered. So we may briefly
-    # get 404 from /, then a 200 with the "starting up" placeholder, then the actual SPA. Poll
-    # page.goto until status==200 (route registered) — Phase 1e exec_in_app pattern: bounded poll,
-    # no bare sleep, raise on persistent failure.
+    # get 404 from /, then a 200 with the "starting up" placeholder, then the actual SPA. The
+    # centralized harness.browser.goto_with_retry helper handles both the status-mismatch retry
+    # AND transient PlaywrightError (net::ERR_*) — F2-3 hardening.
     from playwright.sync_api import sync_playwright
 
     url = f"https://{live_app}/"
@@ -32,29 +32,9 @@ def test_serving_and_editor(live_app, meta):
         try:
             ctx = browser.new_context(ignore_https_errors=True)
             page = ctx.new_page()
-            import time
-
-            from playwright.sync_api import Error as PlaywrightError
-
-            deadline = time.time() + 120
-            resp = None
-            last_status = 0
-            last_err = ""
-            while time.time() < deadline:
-                try:
-                    resp = page.goto(url, wait_until="domcontentloaded", timeout=30000)
-                except PlaywrightError as e:  # net::ERR_*, transient navigation, etc. (F2-3)
-                    last_err = str(e)
-                    resp = None
-                    last_status = 0
-                else:
-                    last_status = resp.status if resp is not None else 0
-                if last_status in (200, 304):
-                    break
-                time.sleep(3)
-            assert resp is not None and last_status in (200, 304), (
-                f"page status {last_status} after polling — n8n route never came up "
-                f"(last error: {last_err or 'none'})"
+            resp = harness_browser.goto_with_retry(page, url, accept_statuses=(200, 304))
+            assert resp is not None and resp.status in (200, 304), (
+                f"page status {resp and resp.status}"
             )
             body = page.content().lower()
             assert "n8n" in body or "<html" in body, "no n8n content served"