Fix Eureka health indicator when registry fetch fails

[pony-maggie]

What changed

Updated the Eureka health indicator so registry fetch failures report DOWN instead of UP when the client has never successfully fetched the registry or the last successful fetch is stale.

Added regression tests for both cases.

Why

A Eureka client that cannot connect to a Eureka server should not report the Eureka health component as UP. This fixes the behavior reported in gh-4562.

How tested

• JAVA_HOME=$(/usr/libexec/java_home -v 17) ./mvnw -pl spring-cloud-netflix-eureka-client -Dtest=EurekaHealthIndicatorTests test -P spring -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn --batch-mode
• JAVA_HOME=$(/usr/libexec/java_home -v 17) ./mvnw -pl spring-cloud-netflix-eureka-client test -P spring -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn --batch-mode

Fixes gh-4562