Generalize the self-host-nightly workflow beyond a single URL

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Problem: The self-host nightly validation workflow is gated on a single configured URL — it assumes exactly one externally-reachable self-host stack to validate against.

Area: ORB / Fleet ops

Proposal: Generalize the workflow to accept a list of URLs (a matrix), running the same validation against each.

Deliverables:

• A workflow that validates N instances, not just one.

AMS Cloud Readiness cross-reference (added 2026-07-12)

See also #5221 in the new AMS Cloud Readiness milestone (#28) — the AMS-side parallel to this exact problem, already citing this issue as precedent. Keep both sides in sync if either design changes.

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JSO-3559

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Releasing from maintainer-only — this milestone is genuinely gated on the undecided 'should we build hosted, and when' question, but this specific issue isn't. Pure CI/testing-tooling generalization (single URL to a matrix of URLs) with direct in-repo precedent (this project's own CI already uses matrix/sharding strategies) — no business decision, no trust/safety-critical production state touched.