chore: Add async contract-test service for FDv1

[jsonbailey]

Summary

Adds the async contract-test service so the SDK contract-test harness can exercise AsyncLDClient. Covers FDv1 (streaming / polling / LDD), hooks, and migration handlers.

What's included

• contract-tests/async_service.py — async client entity + Flask routes for the contract-test harness (FDv1 config, evaluation, hooks, migrations).
• contract-tests/hook.py — hook test support shared by the service.

Held back for a later PR (FDv2)

FDv2 support is deferred to PR 11. The dataSystem (FDv2) branch in AsyncClientEntity.start() (which lazily imported datasourcev2.async_polling / async_streaming) is not included here — it now raises NotImplementedError("FDv2 (dataSystem) is not yet supported in the async contract-test service"). The FDv1 streaming / polling / LDD paths, hooks, and migration handlers are unchanged.

Verification

• async_service.py and hook.py import cleanly (no reference to any held-back datasourcev2.async_* module).
• pycodestyle and isort --check --atomic clean on both files (the repo's mypy lint target runs on ldclient only, not contract-tests).

---

Note

Overview
Adds a parallel async contract-test harness so the shared SDK contract suite can drive AsyncLDClient instead of the sync client. async_service.py exposes the same HTTP surface (status capabilities, create client, per-client commands, teardown) via aiohttp, backed by AsyncClientEntity.

AsyncClientEntity maps harness configuration to AsyncConfig for FDv1 paths: streaming, polling, LDD, events, big segments, and evaluation hooks registered with add_hook() before start(). dataSystem (FDv2) is explicitly rejected with NotImplementedError until a follow-up PR. Command handlers cover evaluation, all-flags state, events, contexts, big-segment status, migrations (AsyncMigratorBuilder with asyncio.to_thread for callback HTTP), and flag / flag-value change listeners.

Supporting pieces: AsyncBigSegmentStoreFixture (async HTTP callbacks to the harness), AsyncListenerRegistry (bridges sync tracker callbacks to async HTTP posts), and AsyncPostingHook in hook.py (async hook stages with asyncio.to_thread for requests.post).

The sync client_entity.py migration callback is updated from Result.error to Result.fail to match the Result API used by migrators.

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