feat(jobs): expose last job failure time

JobActivity 记录最近失败时刻并进入快照；JobRunner 失败时上报该时刻；新增指标 msgx.pipeline.job.last_failure_age_seconds 与健康明细 lastFailureAgeSeconds（对齐 reference 作业健康口径）。

验证：./gradlew test 145 tests / 0 fail / 0 skipped。

src/test/kotlin/com/gzzn/omms/msgexchange/infra/health/JobRunnerHealthIndicatorTest.kt

@@ -59,12 +59,16 @@ class JobRunnerHealthIndicatorTest {
     fun `a failed tick counts up without refreshing the heartbeat`() {
         val activity = JobActivity().apply { started() }
 
-        activity.tickFailed()
-        activity.tickFailed()
+        activity.tickFailed(now.minusSeconds(5))
+        activity.tickFailed(now)
 
         val snapshot = activity.snapshot()
         assertEquals(2L, snapshot.failures)
         assertEquals(0L, snapshot.ticks)
         assertEquals(null, snapshot.lastTickAt)
+
+        val details = details(jobRunnerHealth(snapshot, now))
+        assertEquals(0L, details["lastFailureAgeSeconds"])
+        assertEquals(2L, details["failures"])
     }
 }

src/test/kotlin/com/gzzn/omms/msgexchange/jobs/JobRunnerTest.kt

@@ -69,6 +69,7 @@ class JobRunnerTest {
 
         val snapshot = activity.snapshot()
         assertNull(snapshot.lastTickAt)
+        assertEquals(MutableClock.BASE, snapshot.lastFailureAt)
         assertEquals(0L, snapshot.ticks)
         assertEquals(1L, snapshot.failures)
     }