[Feature]: HTCondorCEBackend — grid CE over the shared `_htcondor` core

[aldbr]

User Story

As the WMS consumer,
I want an HTCondorCEBackend submitting to an HTCondor-CE with IDTOKENS and spooled file
transfer,
So that the second major grid-CE path exists on the new contract, including the destructive-fetch
semantics that motivated IC-ADR-002.

Feature Description

• HTCondorCEBackend: JobBackend + OutputRetriever + Cancellable.
  Deliberately no LoadReporter (DIRAC's getCEStatus is an explicit "not supported") and no
  live diagnostics — capability absence is the point, and the integration suite must show the
  narrowing works.
• Reuses the _htcondor core (submit description, ClassAd parsing, state
  map); differs in transport/auth: remote schedd (-pool/-remote) or CE submission, grid
  universe routing, -spool.
• Token + proxy together per IC-ADR-003 (its Motivation 7 / all-of kinds): SCITOKENS/IDTOKENS
  channel auth (token file + _CONDOR_* env injection, audience <ce>:9619) plus the X.509
  proxy materialised into the submit description (use_x509userproxy = true) — consumed
  site-side (VOMS attributes → APEL accounting), including the
  _CONDOR_DELEGATE_JOB_GSI_CREDENTIALS=false workaround (HTCONDOR-2904). The pure-IDTOKENS
  basic stack config exercises the token path; a proxy-bearing config is the ADR-002 auth axis.
• Destructive retrieval: condor_transfer_data then declared destructive=True
  (HTCondor ≥ 25.8: completed spooled job leaves the queue); evaluate leave_in_queue as the
  re-fetchability override and record the decision. Coarse bounded materialisation (submit-time
  transfer_output_files allow-list, transfer timeout, post-hoc size/count check).
• Phase 2 wiring: contract suite against the htcondor stack's CE path (markers
  remote and htcondor); the fetch-twice destructive_fetch test is the flagship regression
  (versions axis [lts, latest] makes 25.8-class changes red on the Renovate PR).

Definition of Done

• Contract suite green against the htcondor stack CE path with IDTOKENS
• Fetch-twice test asserts the declared one-shot semantics; leave_in_queue decision recorded
• isinstance(be, LoadReporter) is False and the suite's capability-skip path exercises it
• Kill path green; per-id outcomes on partial batches

Alternatives Considered

• Expressing it as BatchBackend(HTCondorCETransport, HTCondorScheduler) — open ADR question;
  blocked on a transport-level "destructive get" notion. Start monolithic, revisit (share only
  _htcondor functions, never a subtype).
• GSI/VOMS auth — out of scope; IDTOKENS-first per ADR-002 §4.

Related Issues

Blocked by: # (_htcondor), #. Validated by: #5 / htcondor stack.

Additional Context