Migrate NodeHealthAPI from homegrown @EndPoint to JAX-RS; add ref guide link

[epugh]

Migrates NodeHealthAPI — the last node-level V2 API still using Solr's homegrown @EndPoint annotation — to standard JAX-RS, following the same pattern as NodeLogging, GetPublicKey, etc.

Design

The logic stays in HealthCheckHandler (minimising diff surface). NodeHealthAPI is a thin JAX-RS wrapper (~60 lines) that delegates entirely to it.

Key changes

• solr/api — New NodeHealthApi interface (@Path, @GET, @Operation) and NodeHealthResponse model (status, message, num_cores_unhealthy)
• NodeHealthAPI — Replaces @EndPoint with JAX-RS; injects CoreContainer, delegates to HealthCheckHandler
• HealthCheckHandler — Logic unchanged; adds public NodeHealthResponse checkNodeHealth(Boolean, Integer) as the shared entry point for both v1 (handleRequestBody) and v2 (NodeHealthAPI); switches to getJerseyResources() / empty getApis()
• V2NodeAPIMappingTest — Removes the now-obsolete @EndPoint/ApiBag routing test for health
• NodeHealthAPITest — New Mockito unit tests for the API class
• NodeHealthAPITest2 — New mock-free integration tests: cloud-mode via real MiniSolrCloudCluster, legacy mode via embedded CoreContainer built from NodeConfig
• implicit-requesthandlers.adoc — Health section now links to both HealthCheckHandler (v1) and NodeHealthAPI (v2) javadocs

[epugh]

Some outstanding questions: 1) there is a more mock and less mock versions of the same test. Which do we prefer? 2) Does this seem like a reasonable pattern for the conversion?

[epugh]

I wish I didnt' have TWO ways of writing tests, one for cloud and one for standalone... sigh.

[epugh]

tests all pass!