The conformance self-test in `libs/checkpoint-conformance` ran every
capability against `InMemorySaver` but asserted only the base ones.
`libs/checkpoint/tests/test_conformance_delta.py`, which ran the delta
conformance tests against it, always skips in CI: `libs/checkpoint`
can't depend on the conformance package, which depends on it. So no CI
job checked `InMemorySaver.get_delta_channel_history` against the delta
conformance tests.
The self-test now asserts every capability `InMemorySaver` implements,
and the skipped copy is gone.
## Tests
- With `InMemorySaver.get_delta_channel_history` broken to return no
history, the self-test now fails on 7 of the 10 delta conformance tests;
on main it still passes.
- `make format`, `make lint` and `make test` pass in
`libs/checkpoint-conformance`.
Add some conformance tests for checkpointer implementations. Includes
additona methods that will be useful if you want to integarte in the
agent server.
