[Subtask] Split core tests into explicit database lanes #13516

Describe the subtask

Split core test execution into four explicit, independent suites: unit, H2, MySQL, and PostgreSQL. This is the first optimization under the database-test epic and establishes clear ownership and evidence before adding finer-grained concurrency.

The change should:

• classify the cross-backend contract explicitly while preserving legacy :core:test behavior for local compatibility;
• add dedicated Gradle tasks for unit, H2, MySQL, and PostgreSQL execution;
• keep every database lane sequential until its mutable state is isolated;
• publish distinct JUnit result and timing artifacts for each lane;
• reconcile normalized test-identity multisets across H2, MySQL, and PostgreSQL;
• merge raw JaCoCo execution data before generating one combined core coverage report; and
• integrate with the build-sharding direction in [#13459] improvement(ci): Shard long-running CI test suites #13495 without duplicating the unit and H2 suites.

Acceptance criteria:

• Each test belongs to the unit suite or the explicit database contract, without duplicate execution across lanes.
• H2, MySQL, and PostgreSQL lanes select exactly one backend and fail closed on unsupported selectors.
• The three database lanes expose the same normalized test-identity multiset.
• CI artifacts make lane identity, results, duration, and coverage inputs independently inspectable.
• The combined core coverage report is generated from the union of raw execution data, not by adding per-lane XML counters.
• Legacy :core:test selection remains compatible during the cutover.

Related public work: #13459, #13495, #13471, and #12584.

Parent issue

#13515