fix exception while loading message cache on startup

[danthe1st]

The current master branch has an issue where the bot can't start due to an issue from #525.

Loading the message cache fails in MessageCacheRepository#getAll because CachedMessage#getAttachments returns an immutable list.

Dec 25 09:26:45 instance-20240715-2322 bash[3890345]: Caused by: java.lang.UnsupportedOperationException                                                                                         
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at java.base@25.0.1/java.util.ImmutableCollections.uoe(ImmutableCollections.java:159)                                              
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at java.base@25.0.1/java.util.ImmutableCollections$AbstractImmutableCollection.addAll(ImmutableCollections.java:165)               
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at net.discordjug.javabot.data.h2db.message_cache.dao.MessageCacheRepository.getAll(MessageCacheRepository.java:90)                
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at java.base@25.0.1/java.lang.reflect.Method.invoke(Method.java:565)                                                               
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:359)                                      
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:196)               
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)                       
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.jav
a:138)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:728)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at net.discordjug.javabot.data.h2db.message_cache.dao.MessageCacheRepository$$SpringCGLIB$$0.getAll(<generated>)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at net.discordjug.javabot.data.h2db.message_cache.MessageCache.<init>(MessageCache.java:81)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at net.discordjug.javabot.data.h2db.message_cache.MessageCache__BeanDefinitions.lambda$getMessageCacheInstanceSupplier$0(MessageCac
he__BeanDefinitions.java:21)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.util.function.ThrowingBiFunction.apply(ThrowingBiFunction.java:68)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.util.function.ThrowingBiFunction.apply(ThrowingBiFunction.java:54)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.beans.factory.aot.BeanInstanceSupplier.lambda$get$2(BeanInstanceSupplier.java:225)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.util.function.ThrowingSupplier.get(ThrowingSupplier.java:58)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.util.function.ThrowingSupplier.get(ThrowingSupplier.java:46)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.beans.factory.aot.BeanInstanceSupplier.invokeBeanSupplier(BeanInstanceSupplier.java:258)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.beans.factory.aot.BeanInstanceSupplier.get(BeanInstanceSupplier.java:225)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.beans.factory.support.DefaultListableBeanFactory.obtainInstanceFromSupplier(DefaultListableBeanFactory.java:
1031)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.obtainFromSupplier(AbstractAutowireCapableBeanFacto
ry.java:1257)
Dec 25 09:26:45 instance-20240715-2322 bash[3890345]:         ... 73 more

For now, the bot is running on commit 6a5b6d3.