feat: wire FDv2 into v3 contract tests, with CI

[beekld]

Adds FDv2 wiring to the server contract-test service so the v3 SDK test harness can exercise the FDv2 code paths, plus a CI job that runs the v3 harness on every change.

• New `dataSystem` block on `ConfigParams` covering harness-provided initializers, synchronizers, and fdv1 fallback.
• Entity manager dispatches FDv2 vs BackgroundSync based on the `dataSystem` presence; when the harness sends synchronizers but no explicit fdv1 fallback, derive one from the synchronizers list (first polling, else first synchronizer; URL routed via global `ServiceEndpoints`).
• `fdv1-fallback` capability advertised by the service.
• Empty `test-suppressions-fdv2.txt`; v3 harness currently passes.
• New CI job runs the v3.0.0-alpha.3 harness; curl-backend variant mirrors the existing v1 contract-test split.