[DT-4061] Add study registration asset read endpoints

[otchet-broad]

Base: branch 6 · 14 files, +524 · Migration: no · UI impact: none

Adds GET /api/dataset/study/{studyId}/assets/{type} for publications,
models, workspaces, presentations, clinicalTrials,
intellectualProperty, and fundingResources, so the study page can read
one asset list at a time instead of pulling the whole registration payload
and picking keys out of the client-managed assets object.

Each endpoint returns the promoted study_property for that type, falling
back to the legacy assets object for a study whose registration has not
been rewritten since the promotion, and returns an empty list for an absent
or malformed value rather than a server error.

Reads go through verifyStudyVisibilityAccess, so assets on a study that is
not publicly visible are readable only by its creator, its custodians, and
admins.

Purely additive: new read-only endpoints, no migration, no change to any
existing response.

Depends on: branch 6 (the StudyAssets helper) and branch 1
(StudyAssetService calls verifyStudyVisibilityAccess).

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Where this sits in the DT-4061 stack

otchet-dt-3990-study-ratings-pi-details was too large to review meaningfully
(95 files, +8168), so it was split into eight PRs. This PR targets otchet-dt-4061-study-asset-fields,
not develop, so its diff shows only its own work.

#	Branch	Base
1	otchet-dt-4061-study-visibility-authz	develop
1b	otchet-dt-4061-study-patch-ownership	otchet-dt-4061-study-visibility-authz
2	otchet-dt-4061-study-pi-details	otchet-dt-4061-study-visibility-authz
3	otchet-dt-4061-study-comments	otchet-dt-4061-study-pi-details
4	otchet-dt-4061-study-dar-metrics	otchet-dt-4061-study-comments
5	otchet-dt-4061-study-recommendations	otchet-dt-4061-study-dar-metrics
6	otchet-dt-4061-study-asset-fields	otchet-dt-4061-study-recommendations
7	otchet-dt-4061-study-asset-endpoints ← this PR	otchet-dt-4061-study-asset-fields

Branch 1b is a sibling rather than a link in the chain: nothing depends on it,
so it can be held or dropped without blocking the others. Land the numbered
chain in order — merging out of order, or squash-merging, will require rebasing
the descendants.

The split is verified lossless: branch 7 plus 1b reproduces the original branch
exactly, apart from five scripts/verify-study-*.sh helper scripts that were
dropped at the author's request. ./mvnw test-compile passes on each branch
independently.

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