refactor: remove block builder in favor of checkpoint builder

[spalladino]

• Remove legacy buildBlock() method from FullNodeCheckpointsBuilder
• Migrate EpochPruneWatcher to use checkpoint-based APIs (startCheckpoint() + buildBlock())
• Add ICheckpointsBuilder and ICheckpointBlockBuilder interfaces to stdlib to avoid circular dependency between slasher and validator-client
• Delete unused IFullNodeBlockBuilder interface and BuildBlockResult type

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/63ecc108bef73a4c�63ecc108bef73a4c8;;�): yarn-project/end-to-end/scripts/run_test.sh compose src/guides/writing_an_account_contract.test.ts (166s) (code: 1) (\033Santiago Palladino\033: refactor: remove block builder in favor of checkpoint builder)