fix(apiserver): map pidinfo from job start into the job model (interactive_job_url)

The worker packs the interactive session URL (ttyd/XJOB) into the start
report's exdata as {"url": ...}; the controller stores it on
job->job.pidinfo->exdata and the capnp Job wire carries pidinfo (pidinfo@9,
JobPIDInfo.exdata@3) — but apiserver's cJobToModel never mapped pidinfo, so
models.JobInfo.InteractiveJobURL (declared only in the query-field whitelist
JobAdditionalFields) had no producer and interactive_job_url was permanently
empty in API responses. xcecli's waitForInteractiveURL/openBrowser flow could
never fire.

- models.JobInfo: add InteractiveJobURL (json interactive_job_url, matching
  the JobAdditionalFields whitelist name).
- convert_pure.go: rawPidInfo (pids/pgids/exdata) + interactiveURLFromExdata;
  rawJobToModel marshals pids/pgids into PIDInfo (pid_info is a JSONBJFields
  entry, parsed by processJobData) and promotes the exdata URL to
  InteractiveJobURL.
- convert.go: cJobToModel extracts job.pidinfo via pidinfo_pid/pidinfo_pgid
  cgo helpers (CGO cannot index the C arrays directly).

Cluster-verified end to end on t1 (fresh apiserver deployed): tty job
(xcecli -I --tty equivalent, exdata {"interactive":1}) now returns
interactive_job_url=http://<worker>:9000 and a populated pid_info; non-tty
jobs return ""/no pid_info. All daemons healthy, zero errors in window.

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

src/frontend/apiserver/backend/convert_pure.go

@@ -526,6 +526,17 @@ type rawRunUsage struct {
 	peakCPU           float64
 }
 
+// rawPidInfo mirrors the C job_pidinfo_t fields used by the Go model —
+// the worker-reported process bookkeeping a job start carries. Exdata is
+// the worker's start_exdata JSON; for interactive jobs (TTY/XJOB/DISPLAY,
+// xce_job_type in src/include/xce/types.h) the worker packs the session
+// URL into it as {"url": ...} (generate_exdata, xcecd_exdata.c).
+type rawPidInfo struct {
+	pids   []int
+	pgids  []int
+	exdata string
+}
+
 // rawJobData mirrors the C xce_job_t fields used by the Go model.
 type rawJobData struct {
 	jobID      int64
@@ -538,7 +549,8 @@ type rawJobData struct {
 	submit     *rawSubmitData
 	execHosts  []rawHostSlot
 	pendReason *rawPendReason
-	runUsage   *rawRunUsage // nil until the job's first usage report
+	runUsage   *rawRunUsage  // nil until the job's first usage report
+	pidInfo    *rawPidInfo   // nil until the worker's start report
 }
 
 // rawWorkerData mirrors the C xce_worker_t fields used by the Go model.
@@ -574,6 +586,26 @@ type rawResourceData struct {
 	interval int
 }
 
+// interactiveURLFromExdata extracts the interactive session URL from a job
+// start's exdata JSON. The worker packs it as {"url": ...}
+// (generate_exdata, xcecd_exdata.c); anything else — non-JSON, other keys,
+// an empty/non-string url — yields "". The URL reaches the apiserver ONLY
+// via this path: job_pidinfo_t.exdata rides the capnp Job wire (pidinfo@9)
+// but was previously dropped by the conversion, leaving the API's
+// interactive_job_url permanently empty and xcecli's openBrowser flow dead.
+func interactiveURLFromExdata(exdata string) string {
+	if exdata == "" {
+		return ""
+	}
+	var obj struct {
+		URL string `json:"url"`
+	}
+	if err := json.Unmarshal([]byte(exdata), &obj); err != nil {
+		return ""
+	}
+	return obj.URL
+}
+
 // rawJobToModel maps a rawJobData to a models.JobInfo.
 // This pure-Go function contains the field-mapping logic and is tested without CGO.
 func rawJobToModel(r rawJobData) *models.JobInfo {
@@ -676,6 +708,20 @@ func rawJobToModel(r rawJobData) *models.JobInfo {
 		info.AvgCPUEfficiency = u.avgCPUEfficiency
 		info.PeakCPUEfficiency = u.peakCPUEfficiency
 	}
+	if p := r.pidInfo; p != nil {
+		// pid_info is a JSONBJFields entry: processJobData parses the JSONB
+		// string before the row leaves the API, so clients see an object
+		// with pids/pgids. The interactive URL is promoted to its own
+		// top-level field — it is what xcecli's waitForInteractiveURL and
+		// the --browse flow consume.
+		if data, err := json.Marshal(struct {
+			Pids  []int `json:"pids,omitempty"`
+			Pgids []int `json:"pgids,omitempty"`
+		}{Pids: p.pids, Pgids: p.pgids}); err == nil {
+			info.PIDInfo = models.Jsonb{String: string(data)}
+		}
+		info.InteractiveJobURL = interactiveURLFromExdata(p.exdata)
+	}
 	return info
 }