Use public aiosendspin client callbacks

[balloob-travel]

Summary

• switch the aiosendspin conformance client adapter off private method overrides
• consume new public SendspinClient callbacks for server/hello, stream/start, and artwork frames
• keep the adapter behavior the same while reducing bespoke integration code

Dependency

This PR depends on the aiosendspin branch that adds the required public hooks:

• codex/aiosendspin-client-callbacks

That branch is not merged yet, so this PR should stay draft until the library change is available on aiosendspin.

Verification

• ./.venv/bin/python scripts/run_all.py --results-dir artifacts/results-aiosendspin-hooks --build-report-path artifacts/build-report-aiosendspin-hooks.json --from aiosendspin --to aiosendspin
• Result: 6 passed, 0 did not pass, 6 total