fix(python): update extension checkout imports

[ShuoRen-TT]

Summary

Fixes #118.

The Python Flower Shop server now imports composed extension Checkout models from the SDK's generated namespace paths under dev.ucp.shopping, matching the current python-sdk output after Universal-Commerce-Protocol/python-sdk#48.

What changed

• Updated AP2, buyer consent, discount, and fulfillment Checkout imports in rest/python/server/models.py.
• Updated the same imports in rest/python/server/integration_test.py.
• Added server_import_test.py to smoke-test importing server.py with the current SDK models.

Why

Current samples main imports Checkout from top-level extension packages such as ucp_sdk.models.schemas.shopping.ap2_mandate. After python-sdk#48, those top-level modules no longer export the composed Checkout; the classes live under ...dev.ucp.shopping. Fresh setup following the README therefore fails before the server starts.

Validation

• Reproduced the original ImportError before the change.
• uv run --directory rest/python/server python - <<'PY' ... import server ... PY
• uv run --directory rest/python/server pytest server_import_test.py
• uv run --directory rest/python/server ruff format --check models.py integration_test.py server_import_test.py
• uv run --directory rest/python/server ruff check models.py integration_test.py server_import_test.py
• uv run --directory rest/python/server python -m py_compile models.py integration_test.py server_import_test.py server.py
• Initialized the Flower Shop sample DB and verified the server starts and GET /.well-known/ucp returns 200 on port 8183.
• Relevant pre-commit hooks passed: ruff, ruff-format, trailing-whitespace, end-of-file-fixer, check-added-large-files, detect-private-key, codespell.

Note: full uvx pre-commit run --files ... could not complete because the Prettier hook failed while installing npm dependency tiny-levenshtein@^1.1.0, unrelated to these Python files.

[ShuoRen-TT]

Hi maintainers, this PR is ready for review.

It fixes #118 by updating the Python sample server to import the generated extension Checkout models from the current SDK namespace. This should also unblock the conformance CI failure currently seen in Universal-Commerce-Protocol/conformance#47.

Local validation is listed in the PR body, including server import, ruff, py_compile, and a real Flower Shop server startup check.

[damaz91]

Thanks for flagging this. We fixed the Python SDK in Universal-Commerce-Protocol/python-sdk#29 - the server should be able to start now, please verify and feel free to reopen if that still doesn't work. Thanks again!