refactor: extract shared BeaconChain test suite

[cats2101]

Summary

Factors out common BeaconChain contract tests into an abstract BeaconChainTestSuite in core/src/testFixtures, so both InMemoryBeaconChainTest and KvDatabaseTest inherit from a single shared suite.

• 12 shared tests cover the BeaconChain interface contract: CRUD for states and blocks, rollback, null lookups, getSealedBeaconBlocks pagination
• Implementation-specific tests remain in their respective classes:
  • InMemoryBeaconChainTest: uncommitted change visibility, byte-array equality, state lookup by block number
  • KvDatabaseTest: persistence across DB restarts, repeated writes, P2P state

Net result: -84 lines of duplicated test code, and KvDatabase now also runs shared tests it previously lacked (e.g. rollback, null lookups, initial state by hash).

Test plan

• ./gradlew :core:test — 16 tests pass (12 inherited + 4 specific)
• ./gradlew :storage:test — 18 tests pass (12 inherited + 6 specific)
• Zero failures, zero errors

Closes #198

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Note

Low Risk
Low risk refactor limited to test code and test-fixture dependencies; production logic is unchanged, with main risk being altered test coverage/assumptions for implementations.

Overview
Refactors duplicated BeaconChain tests into a new shared BeaconChainTestSuite (in core test fixtures) that validates common contract behavior (state/block CRUD, rollback, null lookups, and getSealedBeaconBlocks sequencing).

Updates InMemoryBeaconChainTest and KvDatabaseTest to extend the shared suite and keep only implementation-specific assertions (e.g., uncommitted visibility and byte-array key behavior for in-memory; persistence across restarts for RocksDB). Adds JUnit/AssertJ dependencies to core’s testFixtures so the suite can be reused across modules.

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[cats2101]

Gentle ping — anything else needed here?