Fix auto-configuration to apply a shared container entity manager instance for GraphQLJPASchemaBuilder

[igdianov]

To fix intermittent incorrect empty query results due to Hibernate query result list containing proxied entities when executing query requests concurrently.

The GraphQLJPAQueryBuilder was not using the Shared Entity Manager proxy provided by Spring ORM. Instead, it was configured using a static instance backed by non-thread safe Hibernate Session created by the entity manager factory method.

A Spring shared EntityManager will behave just like an EntityManager fetched from an application server's JNDI environment, as defined by the JPA specification. It will delegate all calls to the current transactional EntityManager already attached to a thread per transaction, if any; otherwise it will fall back to a newly created EntityManager per operation request.

The functional tests worked fine, but not concurrently. It also worked in Hibernate 5 by luck and stopped working in Hibernate 6 after upgrade to Spring Boot 3

[codecov]

Codecov Report

Attention: 54 lines in your changes are missing coverage. Please review.

Comparison is base (d0957ec) 76.11% compared to head (6b83668) 75.53%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
...figure/TransactionalDelegateExecutionStrategy.java	29.54%	31 Missing ⚠️
.../jpa/query/schema/impl/GraphQLJpaQueryFactory.java	51.51%	13 Missing and 3 partials ⚠️
...schema/impl/GraphQLJpaToManyMappedBatchLoader.java	33.33%	3 Missing and 1 partial ⚠️
...GraphQLJpaQueryGraphQlSourceAutoConfiguration.java	88.88%	1 Missing ⚠️
...nfigure/GraphQLSchemaBuilderAutoConfiguration.java	93.33%	1 Missing ⚠️
...jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #380      +/-   ##
============================================
- Coverage     76.11%   75.53%   -0.59%     
- Complexity     1133     1150      +17     
============================================
  Files            75       76       +1     
  Lines          5083     5178      +95     
  Branches        732      739       +7     
============================================
+ Hits           3869     3911      +42     
- Misses          890      940      +50     
- Partials        324      327       +3     

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