Add repository unit tests for Grid and Location

[dmccoystephenson]

Summary

• Adds GridRepositoryImplTest (12 tests) and LocationRepositoryImplTest (17 tests), covering every public method of the two repositories.
• Mirrors the established EntityRepositoryImplTest pattern: @SpringBootTest + @MockBean DbInteractions + a mocked ResultSet. No Testcontainers / no real database — so these run anywhere CI does.
• Cases per method: happy path (single + multiple rows), empty result set, null result set, SQLException handling, and update(...) delegation for the entity_location mutators (addEntityToLocation, removeEntityFromLocation, removeEntityFromCurrentLocation).
• Characterization tests only — they assert current behavior; no production code is touched.

Note on the tracking issue

#129 describes the existing test as a "Testcontainers/JDBC approach." That's inaccurate — EntityRepositoryImplTest actually mocks DbInteractions with Mockito and uses no container. These new tests follow the real sibling pattern. (The cascade-delete / one-entity-per-location scenarios #129 mentions live in the Environment repository, which is deferred below.)

Scope

Partially addresses #129 — Grid + Location only. EnvironmentRepositoryImpl (16 public methods, ~800 LOC of tests) is deferred to a follow-up to stay within the per-PR scope ceiling; #129 remains open to track it.

No Python-client changes: repositories are Java-only JDBC and have no Python mirror.

Test plan

• ./mvnw test -B (JDK 21) — full suite green: 168 tests, 0 failures, 0 errors
• New classes alone: 29 tests, 0 failures
• No production code modified (test-only diff)

Partially addresses #129

[dmccoystephenson]

Self-review rubric (CI green on head — external anchor satisfied):

• Scope: PASS — diff is exactly 2 new files under src/test/java/.../repositories/, 457 insertions, zero production or unrelated changes.
• Tests-new: PASS — all 12 public methods across the two repos are exercised; 29 assertions-bearing tests, all green locally and in CI.
• Tests-fix: N/A — no bug fix in this PR (characterization tests only).
• Sibling structure: PASS — both classes mirror EntityRepositoryImplTest (package, imports, @SpringBootTest + @MockBean DbInteractions, mocked ResultSet, testMethod_Condition naming).
• Sibling renames: N/A — no identifiers renamed.
• Docs: PASS — no API/behavior change, so no doc source-of-truth row is affected.
• Issue resolution: PASS — #129's Grid + Location surface is actually added; PR claims only partial progress, not closure.
• DTO boundary / Spec alignment / Java-Python parallelism / Override: N/A — no controller, endpoint, Python, or override changes.

One issue found (deliberate scoping, not a defect): the SQLException error path is covered for findAll on both repos and for GridRepositoryImpl.findById, but not exhaustively for every finder (e.g. findByEnvironmentId, findByEntityId, findByGridId). The catch blocks there are structurally identical boilerplate, so additional cases would be redundant LOC against an already-over-soft-ceiling diff. Flagging so a reviewer can disagree; happy to expand if exhaustive error-path coverage is preferred.

Summary: green, scope-clean, mirrors the real (Mockito, not Testcontainers) sibling pattern; ready pending review.