fix(builder): persist contractsDB across blocks within a checkpoint

[spalladino]

Motivation

When building multiple blocks within a single checkpoint, the CheckpointBuilder was creating a new PublicContractsDB instance for each block. This meant that contracts deployed in an earlier block within the same checkpoint were not visible to subsequent blocks, causing calls to newly deployed contracts to fail.

Approach

Move the PublicContractsDB instance to be a persistent field on CheckpointBuilder, initialized once in the constructor and shared across all blocks in the checkpoint. Wrap block building in checkpoint/commit/revert semantics on the contracts DB so that failed blocks don't leak state.

Changes

• validator-client: Promote contractsDB from a local variable in makeBlockBuilderDeps to a class field on CheckpointBuilder. Wrap buildBlock in createCheckpoint/commitCheckpoint/revertCheckpoint calls on the contracts DB.
• validator-client (tests): Add tests verifying that the contracts DB checkpoint lifecycle is correctly managed across successful and failed block builds.
• end-to-end (tests): Add e2e test that deploys a contract and calls it in separate blocks within the same slot, validating cross-block contract visibility within a checkpoint.

Fixes A-658