ci: add Python server integration-test and happy-path workflow

[vishkaty]

Follow-up to #118. CI today is lint-only: nothing installs the Python server's dependencies, boots it, or runs the integration tests that already exist at rest/python/server/integration_test.py — so incompatibilities like #118 (the server on main cannot import against python-sdk@main since python-sdk#48) stay invisible until a newcomer follows the README and hits the ImportError.

This adds a workflow that exercises the documented paths end-to-end on every PR/push:

1. uv sync for rest/python/server (with the python-sdk sibling checkout the editable path dependency expects, pinned to the last compatible commit f54858e; the pin comment documents why and how to bump);
2. runs the existing integration tests via pytest (5 tests, currently green);
3. seeds the database from rest/python/test_data/flower_shop per the server README;
4. boots the server and polls /.well-known/ucp for readiness;
5. runs the documented happy-path client against it and asserts its explicit success marker ("Happy Path completed successfully.") — marker-based because the client logs failures without a non-zero exit status.

Had this workflow existed, it would have turned red the day python-sdk#48 merged — exactly the early signal #118 asks for.

Verification: full local dry-run of every step, plus the identical workflow running green in my fork: https://github.com/vishkaty/samples/actions/runs/28898090014

Scope notes

• No changes to pyproject.toml or the README here. The complementary fix for the human setup path (pinning the dependency itself, per Python server won't start when following the README: samples main is incompatible with python-sdk main (ap2_mandate.Checkout removed in sdk #48) #118) is a small follow-up I'm happy to make in whichever direction you prefer (pinned git rev vs released PyPI version).
• Covers rest/python only; the nodejs sample and a2a could get sibling jobs as follow-ups.

[damaz91]

Hello @vishkaty, we are switching to use to ucp-sdk from Pip rather than cloning the python-sdk repo in #131 - this PR would need to be changed once that merges.

I hope this makes sense to you.

[vishkaty]

Reworked per #131 — the workflow now installs ucp-sdk from pip: dropped the python-sdk checkout step and the nested checkout path it required. Verified every step locally end-to-end on ucp-sdk 0.4.3 (server pytest suite + DB init + server boot + happy-path client all green). Thanks for the heads-up @damaz91.