Decide unit or integration for this code

When you are not sure whether to mock the boundary or cross it.

Help me decide what level to test the code below at.

1. Identify the real seams: where the logic is, and where it meets something external (network, database, filesystem, clock, other services).
2. Say which parts deserve fast unit tests, because the logic is intricate and the input space is large.
3. Say which parts must be integration tested, because the bug would live in the interaction rather than in either side. Configuration, serialisation and queries almost always fall here.
4. Name what would be tested twice, and which of the two to delete.
5. Give the approximate shape: how many of each, and what the one end-to-end test should cover if we only get one.

Give a recommendation, not a discussion of the testing pyramid.

Code:

Test strategy. Unit or integration, what to skip, fixture design, and pruning a suite that costs too much.