fix(proxy): resolve critical null-safety, resource leak, and bounds-check bugs

[yasinmiran]

#	File	Bug Type	Fix
1	ProxyController.java	NPE on null md5	Null guard before checksum validation
2	PublishMQAspect.java	Unsafe cast	instanceof checks with pattern matching
3	PublishMQAspect.java	Null attributes	Null checks before .toString()
4	CredentialsMappingAspect.java	Null attributes	Null checks before .toString()
5	TokenService.java	Array bounds	Validate JWT has 3 parts
6	HeartbeatService.java	Assert in production	Replace assert with if + early return
7	AAIAspect.java	Array bounds	split(":", 2) + length check
8	LocalEGATSDProxyApplication.java	Resource leak	try-with-resources