feat(core): single registerSingleton to avoid duplication

[seregamorph]

There is a scenario when ContextDiagnosticApplicationInitializer is specified in both spring.factories discovered in the classpath and specified explicitly on the class. In this case test execution fails with

19:23:47 [main] ERROR org.springframework.boot.SpringApplication - Application run failed
java.lang.IllegalStateException: Could not register object [ContextDiagnostic[contextLoadStartTime=1754853753835, contextLoadEndTime=1754853827194, contextLoadComplete=true, heapMemoryUsedBytes=134002912, nonHeapMemoryUsedBytes=60453496, availableProcessors=12, totalMemoryBytes=226492416, maxMemoryBytes=6442450944, freeMemoryBytes=92055928]] under bean name 'contextDiagnostic': there is already object [ContextDiagnostic[contextLoadStartTime=1754853753830, contextLoadEndTime=1754853827194, contextLoadComplete=true, heapMemoryUsedBytes=134002912, nonHeapMemoryUsedBytes=60432104, availableProcessors=12, totalMemoryBytes=226492416, maxMemoryBytes=6442450944, freeMemoryBytes=92055928]] bound
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.registerSingleton(DefaultSingletonBeanRegistry.java:124)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.registerSingleton(DefaultListableBeanFactory.java:1175)
	at digital.pragmatech.testing.diagnostic.ContextDiagnosticApplicationInitializer.lambda$initialize$0(ContextDiagnosticApplicationInitializer.java:34)

and this failure is pretty unclear as test eventually fails with other diagnostics (if the suite is heavy and big). This simple fix makes the singleton registration conditional to do it not more than once.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[seregamorph]

alternatively, this could be implemented as

if (!applicationContext.getBeanFactory().containsSingleton(BEAN_CONTEXT_DIAGNOSTIC)) {
  applicationContext
      .getBeanFactory()
      .registerSingleton(BEAN_CONTEXT_DIAGNOSTIC, completedDiagnostic);
}

but this solution with equals looks as a proper solution

[rieckpil]

good catch, thank you!