fix(traefik): a router's 5xx share is judged only over enough requests, like its 4xx share (#5859)

Two 500s of about sixteen requests graded roundtable@swarm critical at
9.1%. The Settings → Thresholds minimum of requests in 15 minutes (30 by
default) now guards both shares; the 5xx share still keeps its fixed
cutoff whatever the router's history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/traefik/test_normals.py

@@ -1,8 +1,8 @@
 """A router's 4xx share and p95 are graded against its own normal once it has
 one (#5741, System 182): a router over the fixed cutoff but inside its normal
 is healthy and says so; one out of its normal is a warning that says what is
-usual. The 5xx share keeps its fixed cutoff, and a 4xx share over too few
-requests isn't graded."""
+usual. The 5xx share keeps its fixed cutoff, and neither share over too few
+requests is graded."""
 from types import SimpleNamespace as NS
 
 from plugins.traefik.routes import STATE_WINDOW_WORDS, grade_routers, router_state
@@ -60,15 +60,18 @@ def test_without_a_normal_the_fixed_cutoffs_still_apply():
     assert router_state(_window(p95=450.0), TH, "")[0] == "warn"
 
 
-def test_5xx_keeps_its_cutoff_whatever_the_history_or_traffic():
+def test_5xx_keeps_its_cutoff_whatever_the_history():
     usual_5xx = {"error_rate_4xx_pct": _norm(50.0)}
     assert router_state(_window(e5=40.0), TH, "", usual_5xx)[0] == "crit"
-    assert router_state(_window(e5=40.0, requests=4.0), TH, "")[0] == "crit"
+    assert router_state(_window(e5=40.0, requests=300.0), TH, "", usual_5xx)[0] == "crit"
 
 
-def test_a_4xx_share_over_a_handful_of_requests_is_not_graded():
+def test_a_share_over_a_handful_of_requests_is_not_graded():
     assert router_state(_window(e4=33.0, requests=3.0), TH, "") == ("ok", None)
     assert router_state(_window(e4=33.0, requests=300.0), TH, "")[0] == "warn"
+    # roundtable@swarm on 2026-10-15: two 500s of about sixteen requests (#5859)
+    assert router_state(_window(e5=12.5, requests=16.0), TH, "") == ("ok", None)
+    assert router_state(_window(e5=12.5, requests=30.0), TH, "")[0] == "crit"
 
 
 def test_grade_routers_hands_each_router_its_own_normals():