test(core): enforce the BaseEntity equals and hashCode contract

[philipgreat]

Background

BaseEntity.equals() compares ID and type name, while hashCode() currently also includes version. Equal entities with different versions may therefore produce different hash codes, which breaks hash-based collections.

Scope

Add focused equality-contract tests and make the smallest correction required.

Test cases

• Same instance is equal to itself.
• Same concrete entity type and ID are equal.
• Equal entities have identical hash codes even when versions differ.
• Different IDs are not equal.
• Different concrete entity classes are not equal.
• HashSet lookup succeeds for an equal entity instance.

Acceptance criteria

• The tests demonstrate the current contract violation before the correction.
• equals() and hashCode() use the same identity fields after the fix.
• No persistence or runtime fixture is involved.