Faithful BIDS-inheritance staging test via a splice-point pre-hook

[asmacdo]

Depends on: #381 (splice-point hooks) landing first.

Problem

The BIDS-inheritance grab added to participant_job.sh.jinja2 (the fixup on #376) is partially covered today:

• Render/structure unit tests — assert the generated script text is correct (root tier resolved for every job, subject tier only for session jobs, zipped inputs skipped, common_paths threaded). These do not exercise the runtime resolve_tier (git ls-tree) logic.
• A one-off manual assertion — a temporary block in the job template that prints the staged tiers and asserts dataset_description.json, run once to confirm staging (validated via the e2e walkthrough, then reverted — too BIDS-specific to bake into every production job).
• The fixup removed explicit dataset_description.json from common_paths (it now defaults to []), so the e2e walkthrough fails if the grab doesn't stage it — an implicit but real regression guard.

Proposal

Once the splice-point hooks (#381) merge, we can do better. By using a pre-app hook we can add arbitrary assertions that run inside the job, i.e. this or that file is present and not a broken symlink. The hook can exit 0, bypassing the container run altogether. I suspect this could be a strong test pattern that could eventually speed up / strengthen existing tests (not in scope here though).

Assertions to make (session-level run, on a fixture or a small real dataset):

• Positive: dataset_description.json and the root-tier inherited sidecars (e.g. T1w.json, a task-*_bold.json, participants.tsv) are present in the per-job working tree.
• Positive (session): the subject-tier files (e.g. sub-XX/sub-XX_sessions.tsv) are present for a sub-XX/ses-YY job.
• Negative: another subject's directory (sub-other/) is absent; for a session job, the subject's other sessions (sub-XX/ses-other/) are absent — proving the non-recursive root anchoring + sparse-checkout restriction.

Notes

• The render/structure unit tests and the config-default test stay as the fast first line; this hook test is the integration layer.