test(checkpoint): replace vacuous interface tests with behavioural co…

[N-otorious]

Replace vacuous checkpoint tests with behavioural coverage

Removes three interface-shape tests that exercised no runtime code and replaces them with 19 behavioural tests
against a mocked Prisma client, covering hasCheckpoint, getLastCheckpoint, commitBatch (fresh insert, replayed
batchId, transaction failure), and updateAccountSummaries. checkpoint.ts moves from 0% to 100%
statement/function/line coverage.

Note: commitBatch and hasCheckpoint are not yet wired into startIndexer().

[Miracle656]

#229 has merged, and this is the same work on the same file — a second, longer version of src/__tests__/checkpoint.test.ts. Merging this now would replace what just landed (274 insertions against 184 deletions on that file), not add to it.

I am not going to close one of your PRs for you. Tell me which you want:

• Keep test(checkpoint): replace vacuous interface tests with behavioural co… #229 and close this — simplest, and test(checkpoint): replace vacuous interface tests with behavioural co… #229 is already in.
• Or rebase this on current main so the diff shows what it adds on top of test(checkpoint): replace vacuous interface tests with behavioural co… #229 rather than instead of it, and I will review that.

If this version has assertions #229 lacks, the second option is the better one and I would like to see them. From a skim the extra length is mostly the file header and structure rather than new cases, but you know the difference better than I do.

For what it is worth, the merged version earns its place for the right reason: it asserts what was passed, not just what came back —

expect(callArg.where.network_batchId.network).toBe("mainnet")

That is the assertion that catches a helper returning the right shape from the wrong network. We fixed exactly that class of bug in the indexer today: mainnet was reading a testnet contract, found no events, and reported itself healthy.