CVE-2026-81515: Steeltoe.Discovery.Eureka: malformed enum/bool/timestamp field aborts entire registry fetch (DoS)

## Summary Steeltoe's Eureka discovery client deserializes the registry response as a single unit. If any registered instance contains a field value that cannot be parsed (for example, an unrecognized `actionType`, a non-boolean value for `isCoordinatingDiscoveryServer`, or a non-numeric timestamp), deserialization of the entire registry fails. All Steeltoe Eureka clients connected to that registry then receive an empty or stale instance list until the malformed registration is removed. This is a related but distinct issue from [CVE-2026-50196](https://github.com/advisories/GHSA-j8ph-6fxj-g533), which addressed only the `DataCenterInfo.name` field. ## Impact Any principal that can register or update a service instance in the Eureka registry can trigger a service-discovery outage for all Steeltoe applications connected to the same registry. The outage persists until the offending registration is removed. ## Affected configuration * Application uses `EurekaDiscoveryClient` (any deployment, including self-hosted Eureka). * The registry contains at least one service instance with an unrecognized value in a field such as `actionType`, `status`, `isCoordinatingDiscoveryServer`, or timestamp fields. * Mixed-platform environments — for example, Java or Spring services co-existing in the same Eureka registry — are more likely to encounter this condition. ## Mitigations If an immediate upgrade is not possible: * Audit the Eureka registry for registrations containing non-standard field values, particularly those originating from non-.NET clients. * Restrict write access to the Eureka registration API to trusted services.