Optimize object mapping for value types

[zantvoort]

Problem

ObjectMapperFactory.getObjectMapper has no dedicated path for "leaf value types" like UUID, String, BigDecimal, boxed primitives, and java.time.*. When a query maps to one of these as a scalar (e.g. query.getSingleResult(UUID.class)), the factory falls through every special case and lands in the constructor-scan loop:

  for (Constructor<?> constructor : type.getDeclaredConstructors()) {
      if (constructor.getParameterTypes().length == columnCount) {
          return Optional.of(wrapConstructor(constructor));
      }
  }

It then calls setAccessible(true) on the first matching constructor. For UUID.class with columnCount == 1, the only 1-arg constructor is the private UUID(byte[]) in java.base, and the JDK module system refuses access:

  java.lang.reflect.InaccessibleObjectException: Unable to make private java.util.UUID(byte[]) accessible:
  module java.base does not "opens java.util" to unnamed module @685f4c2e
      at st.orm.core.template.impl.ObjectMapperFactory.construct(ObjectMapperFactory.java:172)

For other leaf types the same path "works" only by luck of getDeclaredConstructors() ordering — e.g. String.class happens to land on the public String(String) copy-constructor, and we pay one allocation per row to wrap a value that's already correct. For BigDecimal.class and others, behavior depends on whichever 1-arg constructor the JVM returns first.

Affected APIs

Any call that maps a single column to a leaf type, including:

• Query.getSingleResult(UUID.class) / getResultStream(UUID.class)
• Query.getRefStream(EntityType.class, UUID.class) (PK type lookup)
• Single-column projections / aggregates with these target types

Proposed fix

Add a ValueMapper next to PrimitiveMapper / EnumMapper that provides a single-column pass-through for types QueryImpl.readColumnValue already returns directly:

• Boxed primitives: Boolean, Byte, Short, Integer, Long, Float, Double
• String, BigDecimal, ByteBuffer, UUID
• java.util.Date, Calendar, java.sql.Date, Time, Timestamp
• LocalDateTime, LocalDate, LocalTime, Instant, OffsetDateTime, ZonedDateTime

Dispatch to ValueMapper.getFactory(columnCount, type) in ObjectMapperFactory.getObjectMapper before the constructor-scan fallback. The mapper just returns args[0] — no reflection, no allocation, no module-access issues.

Benefits

• Fixes InaccessibleObjectException for UUID and any other JDK type whose 1-arg constructors are private.
• Removes brittle reliance on getDeclaredConstructors() ordering for String, BigDecimal, etc.
• Eliminates one allocation per row for String.class queries (no more new String(s)).
• Aligns scalar value mapping with what readColumnValue already produces.

Test coverage

Add regression tests under EntityRepositoryIntegrationTest (or a new dedicated test) for:

• getSingleResult(UUID.class) on a single UUID column.
• getResultStream(UUID.class) on a multi-row UUID column.