feat(alerts): Send test on a rule checks that its alerts reach someone (#5466)

#5466 "Send test: check a rule's delivery from the rule". A rule's page gets
a Send test button (operator role), and MCP gets send_test_alert. Each sends
a TEST alert for that rule now, by email to everyone who gets alerts and by
webhook. The alert carries the rule's name and condition, at the latest
reading of what it watches; a rule covering hosts uses its first host. The
result for each channel shows under the button, or "Nothing is set up to
send to". Nothing fires, no event is written, and the test is audited as
alert_rule.tested. An alert with no reading now says "has no reading yet"
instead of "is None".

Docs: alerting.md also gains the burst email from #5465 "One email per burst
of alerts".

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

tests/core/test_mail.py

@@ -96,11 +96,13 @@ def test_without_steward_s_address_there_is_no_link_and_utc_is_said_once():
 
 def test_each_state_reads_as_a_sentence():
     head = {s: mail.alert_facts({**ALERT, "state": s, "value": v}, timezone.utc)["headline"]
-            for s, v in (("FIRING", 93.5), ("RESOLVED", 71.0), ("NO_DATA", None), ("DATA_RESUMED", 80.0))}
+            for s, v in (("FIRING", 93.5), ("RESOLVED", 71.0), ("NO_DATA", None), ("DATA_RESUMED", 80.0),
+                         ("TEST", None))}
     assert head == {"FIRING": "Disk used on web01 is 94%.",
                     "RESOLVED": "Disk used on web01 is back within its limit, at 71%.",
                     "NO_DATA": "Steward has had no disk used reading from web01.",
-                    "DATA_RESUMED": "web01 is reporting disk used again."}
+                    "DATA_RESUMED": "web01 is reporting disk used again.",
+                    "TEST": "Disk used on web01 has no reading yet."}
     down = mail.alert_facts({**ALERT, "metric": "is_up", "operator": "<", "threshold": 0.5, "value": 0.0},
                             timezone.utc)
     assert (down["headline"], down["condition"]) == ("web01 is down.", "is down")