Consider HA connection state for Vault standby health

[henkhofs]

Spring Cloud Vault's HealthBuilderDelegate currently reports any unsealed standby Vault node as UP. Vault's sys/health response includes ha_connection_healthy for standby nodes; a standby without a healthy HA connection should not be considered healthy by actuator health.

Context:

• Vault returns 429 for standby sys/health responses by default, with a response body containing standby=true and ha_connection_healthy.
• Spring Vault PR Handle Vault standby health responses spring-projects/spring-vault#1007 exposes ha_connection_healthy through VaultHealth and aligns reactive sys/health error-body handling with the blocking implementation.
• Once Spring Cloud Vault consumes a Spring Vault version with that field, HealthBuilderDelegate could treat standby health as UP only when ha_connection_healthy is true, and DOWN otherwise.

Suggested semantics:

• initialized=false -> DOWN
• sealed=true -> DOWN
• standby=true && ha_connection_healthy=true -> UP
• standby=true && ha_connection_healthy!=true -> DOWN
• performance_standby=true -> UP
• active -> UP