xcecli: fix job list "cannot unmarshal pending_reason" + verbose fields

apiserver now returns pending_reason as an object ({main_reason,
main_reason_name, host_reasons, res_reasons}) since the PEND-reason work.
xcecli's QueryJob.PendingReason was a plain string, so every job list
failed with "cannot unmarshal object into string". Two fixes:

- xcecli/models/jobs.go: PendingReason is now a string-backed type with a
  custom UnmarshalJSON that accepts null / empty string / plain string
  (older API) / the object form (current API, rendered as readable text —
  LSF bjobs -p style: "match_fail; host t1: match_fail; ..."). Unknown
  objects fall back to the raw bytes instead of erroring, so a job list
  can never fail on an unexpected pending_reason shape.
- apiserver/models/jobs.go: JobAdditionalFields gains PendingReason so the
  fields= query (xcecli `jobs list -l`) no longer rejects pending_reason
  with "unsupported query field".
- xcecli/common/output/page_print_jobs.go: adapt the verbose printer to the
  new type.

Tests: xcecli/models/jobs_test.go — 6 cases (null / empty / plain string /
object / within QueryJob / unknown object). xcecli 9 packages + apiserver
models/controllers/utils all pass.

Verified on the test cluster: `xcecli jobs list` (no unmarshal error) and
`xcecli jobs list -l` show "PENDING REASONS: host_unavail; host t1:
host_unavail; host t2: match_fail; host t3: match_fail".

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

src/frontend/apiserver/models/jobs.go

@@ -613,6 +613,7 @@ type JobAdditionalFields struct {
 	Pids              []int     `json:"pids"`  // List of process IDs associated with the job
 	Pgids             []int     `json:"pgids"` // List of process group IDs associated with the job
 	InteractiveJobURL string    `json:"interactive_job_url"`
+	PendingReason     Jsonb     `json:"pending_reason"` // PEND reason (scheduler-set, JSONB object)
 }
 
 type JobSubmitRequest struct {