apiserver: apply the controller's jobPidInfo start-report push

handleNotifyData had no case for NotifyData_data_jobPidInfo, so the
controller's start-report push (apiserver_job_start_fun, fired from
HOOK_DETAIL_JOB_START the moment WAL_JOB_START commits) was silently
dropped. interactive_job_url then stayed invisible until the next
periodic rusage-driven job UPDATE re-delivered the full row — up to
10s later (job_rusage_collect_interval), the dominant chunk of an
-I --tty job's submit-to-first-output latency (measured ~6s on t1,
server-side work itself is ~0.6s).

Add OnJobPidInfo to the notification interface and DefaultHandler:
it merges the worker-reported pids/pgids and the exdata {"url": ...}
onto the already-stored job record (get-mutate-put, mirroring
OnJobFinished) and publishes a job.updated event. The CGO decode
side is factored into cPidInfoToRaw/pidInfoJSONB, shared with the
existing xce_job_t conversion.

Cluster-verified on t1: URL is pollable ~0.6s after submit
(previously ~11s), full submit/run/finish lifecycle clean, all
daemons healthy.

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

src/frontend/apiserver/backend/handler.go

@@ -115,6 +115,44 @@ func (h *DefaultHandler) OnJobFinished(jobid uint64, exitCode int) {
 		zap.Int("job_status", existing.JobStatus))
 }
 
+// OnJobPidInfo applies a controller-pushed job start report
+// (NotifyData_data_jobPidInfo, fired from HOOK_DETAIL_JOB_START after
+// WAL_JOB_START commits) onto the already-stored job record: the worker's
+// pids/pgids become PIDInfo, and an interactive job's exdata {"url": ...}
+// becomes InteractiveJobURL — the field xcecli's waitForInteractiveURL
+// polls. Without this handler the push was silently dropped by
+// handleNotifyData and the URL stayed invisible until the next periodic
+// rusage-driven job UPDATE re-delivered the full row (up to 10s later,
+// job_rusage_collect_interval).
+//
+// Mirrors OnJobFinished's get-mutate-put-publish shape: the full job row
+// (submit spec, state, timestamps) always arrives ahead of or with the
+// start report — SUBMIT via OnJobCreated/join push, RUN state itself via
+// the dispatch-time update — so a jobid not in the store is logged and
+// dropped rather than inserted half-empty.
+func (h *DefaultHandler) OnJobPidInfo(jobid uint64, pidInfo *rawPidInfo) {
+	existing, ok := store.Jobs.Get(jobid)
+	if !ok {
+		logger.Info("OnJobPidInfo: jobid not found in store, ignoring",
+			zap.Uint64("jobid", jobid))
+		return
+	}
+	if pidInfo != nil {
+		if jb, ok := pidInfoJSONB(pidInfo.pids, pidInfo.pgids); ok {
+			existing.PIDInfo = jb
+		}
+		existing.InteractiveJobURL = interactiveURLFromExdata(pidInfo.exdata)
+	}
+	store.Jobs.Put(jobid, existing)
+	DefaultBroker.Publish(Event{Type: EventJobUpdated, JobID: jobid,
+		Status: models.JobStateStr(existing.JobStatus), Job: &existing})
+
+	logger.Info("OnJobPidInfo: start report applied",
+		zap.Uint64("jobid", jobid),
+		zap.Int("pids", len(pidInfo.pids)),
+		zap.String("interactive_job_url", existing.InteractiveJobURL))
+}
+
 func (h *DefaultHandler) OnQueueUpdated(queue *models.QueueInfo) {
 	store.Queues.Put(queue.QueueName, *queue)
 }