test(core): cover EntityKey equality, hashing, ordering, and string representation

[philipgreat]

Background

EntityKey is used as the key for TreeMap and TreeSet collections in the change-tracking ledger, but teaql-core currently has no unit tests.

Scope

Add a focused EntityKeyTest.

Test cases

• Keys with the same entity type and ID are equal and have the same hash code.
• Different entity types or IDs are not equal.
• Natural ordering sorts first by entity type and then by ID.
• A null ID sorts consistently before non-null IDs of the same type.
• A null entity type is rejected.
• toString() renders the stable type:id form.

Acceptance criteria

• Tests run in the teaql-core module with JUnit 4.
• Tests cover only EntityKey; no production behavior is changed unless a failing contract test reveals a defect.