[codex] Add generated SDK import smoke test

[ivanleomk]

Summary

Adds a lightweight unittest smoke test for generated SDK import integrity.

The test verifies that:

• import kagglesdk succeeds
• every discovered kagglesdk.* module imports cleanly
• KaggleClient() instantiates
• the expected top-level client groups are present
• the generated client exposes 21 nested *_client subclients

Why

The 0.1.31/0.1.32 import regression was caused by generated files referencing modules that were not included in the synced/published package. A build-only release step can still succeed in that case because wheel construction does not import the full transitive module graph.

This gives the release flow a cheap import-closure check that catches missing generated modules before publishing.

Evidence

Current main plus this test fails in a fresh Python 3.13 venv after pip install -e .:

ModuleNotFoundError: No module named 'kagglesdk.competitions.legacy'

Ran 2 tests in 1.218s
FAILED (errors=2)

The same test overlaid onto origin/fix-imports-0.1.33 passes in a fresh Python 3.13 venv after pip install -e .:

Ran 2 tests in 1.852s
OK

Commands used:

/opt/homebrew/bin/python3 -m venv .venv
.venv/bin/python -m pip install -q --upgrade pip
.venv/bin/python -m pip install -q -e .
.venv/bin/python -m unittest tests.test_import_smoke

[ivanleomk]

Superseded by #61 from branch import-smoke-test.