Guard EntityFactory against null ResultSet and -1 id sentinel

[dmccoystephenson]

Summary

EntityFactory implemented the same "get next id from a sequence, then INSERT" pattern as EnvironmentFactory but omitted two safety checks the latter performs, leaving two defects on the live entity-creation path (POST /api/v1/entities/{name} and DebugController):

1. Unguarded null ResultSet → NPE. getNextEntityId() called rs.next() without checking rs. DbInteractions.query(...) returns null on failure, so a failed/locked query threw NullPointerException instead of returning -1. Fixed with if (rs == null) return -1; (matches EnvironmentFactory).
2. Unchecked -1 sentinel → bad INSERT. createEntity() never checked id == -1 before building the INSERT, so on failure it would write a row with entity_id = -1. Fixed with an if (id == -1) guard that logs and throws EntityCreationException (matches EnvironmentFactory).

Tests

Adds EntityFactoryTest (mocks DbInteractions, mirroring the repository tests' @SpringBootTest + @MockBean style):

• success path returns the entity;
• null-ResultSet → throws, and asserts no update/INSERT is attempted;
• nextval returns no row (-1) → throws, and asserts no update/INSERT is attempted;
• INSERT failure → throws.

Both guard tests were confirmed to fail without the fix (NPE on the null case; the -1 case attempts the INSERT and trips the never().update(...) verification).

Test plan

• ./mvnw test — 220/220 pass (was 216; +4), JDK 21
• Regression demonstrated: reverting EntityFactory.java to main makes the two guard tests fail
• Python client — n/a (no src/main/python change)

Closes #138

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