The publish workflow installed only SDK runtime dependencies and invoked
unittest discovery, causing the pytest-based conformance module to fail
import. Installing pytest alone would still leave function-based tests
unexecuted by unittest.

Declare a shared test extra, use it in both workflows, and run pytest in
the publish verification job. Document the same local test commands.

Validation: reproduced the original import failure in a clean virtual
environment, then installed the test extra and passed all 125 Python tests.

Co-authored-by: Codex <noreply@openai.com>