fix: avoid stringifying entire user_properties

[LouisLau-art]

Fixes #12.

• Make cleanup_unserializable sanitize nested values so a single unserializable value (e.g. a set inside user_properties) doesn't stringify the whole user_properties payload.
• Add a regression test covering the user_properties case.
• Skip the existing subtests integration test when pytest-subtests isn't installed.

Test:

• python -m pytest