fix(observability): tolerate missing health contributors in readiness group

Default readiness group includes 'db,diskSpace'. Apps without a JDBC/r2dbc
autoconfig (e.g. callbacks-web tests, notification adapters) crash startup
with 'Included health contributor db does not exist'. Set
management.endpoint.health.validate-group-membership=false so missing
contributors are silently dropped.

Author: Andrés Contreras Guillén

src/main/resources/application-firefly-observability.yml

@@ -48,6 +48,9 @@ management:
       show-components: always
       probes:
         enabled: true
+      # Required so apps without a db (e.g. callbacks-web non-r2dbc tests) do not
+      # crash startup when "db" is missing from the readiness group.
+      validate-group-membership: false
       group:
         liveness:
           include: livenessState