Ship the library in the sdist, and stop shipping the tests

[QuiteYellow]

tests/ has been in the sdist include list since the commit that introduced packaging, with no stated reason, and against the wheel comment in the same file: mqtt_demo/, tests/ and setup_cert.py are in-repo development material.

Nothing had ever run them from an sdist, because the packages they import stayed behind. From a bare extraction:

• two files failed at collection, importing tools/;
• five tests failed, reading docs/;
• seven more failed at collection, importing mqtt_demo or setup_cert.

CI never noticed, because its sdist job installs the artefact and imports one module.

Why this does the opposite of shipping them

The first version of this branch shipped tools/ and docs/ so the tests would run. That let test placement decide what the package distributes, which is backwards. It also put documentation-hygiene checks in a packager's path: whether docs/api.md matches the code, and whether this repository's markdown links resolve, has no bearing on a build of the library.

So the sdist now carries the library and its metadata, which is what the file already said it intended. check_distribution.py enforces that set.

What this costs

Nothing identified. The wheel was already library-only. The repository suite still runs 798. CI's sdist job installs the artefact and imports from it exactly as before, verified against the new one. Anyone wanting to run the suite uses the git tag, as CI does.

The sdist halves, 213501 bytes to 107177.

Validation

798 tests pass on Python 3.11, 3.13 and 3.14, and on the dependency floor. check_distribution.py passes on both artefacts, and check_share_safety.py is clean.