Add standard "known vulnerabilities" metadata field, sourced from OSV #1360

Is your feature request related to a problem? Please describe.

pulp_python already has vulnerability-scanning capability: a scan action on PythonRepositoryVersionViewSet (added in #1013/#1012) queries OSV.dev and stores results in pulpcore's VulnerabilityReport model. However, that data is not exposed through the standard PyPI "known vulnerabilities" metadata field described in the PyPI JSON API spec — the existing scan output lives in its own report resource, not in the Simple API / JSON API response shape that standard PyPI tooling expects, and today's scan is manual/on-demand only. Tooling that relies on the standard field for vulnerability visibility (e.g. Red Hat Trusted Libraries / the pulp-trustify effort) has no way to consume it from pulp_python today.

Describe the solution you'd like

Surface the existing VulnerabilityReport.vulns data (see above) through the Simple API / JSON API responses as the standard vulnerabilities field, following a comparable per-(name, version) marker pattern to native PackageYank support (#1282) for the API-shape/serializer side. OSV remains the data source underneath — no new scanning backend needed. Open design question to resolve here: scanning today is manual/on-demand only — deciding whether it should also trigger automatically (e.g. on sync/upload) is in scope for this issue.

This is agreed phase-1 scope for the Pulp-Trustify integration effort — complex/custom scanning and Trustify-specific data-source integration are intentionally deferred to a later phase (see context below).

Describe alternatives you've considered

• Implementing this in pulp-service instead — ruled out; pulp-service (the hosted Pulp team) is an external stakeholder for this effort, not a place for this logic to live.
• Full Trustify-backed scanning/library integration now — deferred; this issue is scoped narrowly to the standard OSV-sourced vulnerabilities field, decoupled from the broader pulp-trustify core-library work happening in parallel in pulp/pulp_trustify.
• Building a new scanning mechanism from scratch — ruled out; Implements the vulnerability report #1013/Add vulnerability reports to python plugin #1012 already added an OSV.dev-backed scan pipeline (VulnerabilityReport model, scan action) that this issue should extend rather than duplicate.

Additional context

• Part of the broader Trustify integration program (Red Hat Trusted Libraries / Calunga); tracked upstream at pulp/pulp_trustify.
• Related PRs: Implements the vulnerability report #1013/Add vulnerability reports to python plugin #1012 (existing OSV.dev-backed scan pipeline — VulnerabilityReport model, scan action — the mechanism this issue extends), [PULP-2041] Add PEP 592 yank support #1282 (native PackageYank content model — API-shape precedent for surfacing a per-package marker).
• Filed on behalf of Otávio Fernandes; Gerrod Ubben to be notified separately once this is actually filed — he may pick it up next sprint, or review a contribution from Otávio.