feat: Add AsyncLDClient with FDv1 data system and public API

[jsonbailey]

Summary

Adds the async LaunchDarkly client (AsyncLDClient) built on asyncio/aiohttp, wired to the FDv1 data system, plus the public API surface for constructing it. This is the first extraction slice from the async SDK implementation branch (epic SDK-60).

The client uses an explicit lifecycle: construct with AsyncLDClient(config), then await start() / await close() (or async with). Construction is loop-free, so the client can be built before an event loop exists and started per worker (e.g. one loop per ASGI worker).

Lifecycle shape

• __init__ builds the object graph, loop-free — data system, status providers, big-segment manager, evaluator, and flag tracker are all constructed here; no event loop is required.
• start() does the loop-bound work — creates the shared aiohttp session (lazily, on first use inside the loop), starts the data source, starts the big-segment status poll, and sets up the event processor.
• flag_tracker and the status providers are available before start(), matching the other LaunchDarkly SDKs (they no longer raise). Flag-change listeners registered before start() fire once data begins to flow.
• The shared HTTP session is created only when a network component first needs it, so offline/LDD mode creates none.

What's included

• ldclient/async_client.py — AsyncLDClient with the FDv1 data system path.
• ldclient/impl/datasystem/async_fdv1.py — async FDv1 data system (resolves the HTTP session from a lazy provider at start).
• ldclient/impl/datasystem/__init__.py — adds the AsyncDataSystem protocol.
• ldclient/impl/async_flag_tracker.py — the flag tracker's callback scheduler is created lazily on first use, so the tracker can be built without a running loop.
• ldclient/impl/async_big_segments.py — the status poll task is created in __init__ and started in start().
• ldclient/__init__.py, ldclient/client.py, ldclient/impl/client_common.py, ldclient/impl/stubs.py — public API / shared plumbing needed by the async client (ldclient.__getattr__ lazy-loads AsyncLDClient so plain import ldclient doesn't pull in aiohttp; get_plugin_hooks now takes config.plugins; async null event/update processor stubs).
• ldclient/testing/mock_async_components.py, ldclient/testing/stub_util.py — async test doubles.
• ldclient/testing/test_async_client.py, ldclient/testing/impl/test_async_big_segments.py, ldclient/testing/test_sync_async_parity.py — coverage for the async client, the big-segment lifecycle, and sync/async public-API parity.

Held back for a later PR (FDv2)

This slice is FDv1-only. The FDv2 branch in _make_data_system() raises NotImplementedError("FDv2 is not yet supported in the async client"); the FDv2 modules (async_fdv2, datasourcev2.async_*) and their session wiring land in PR 11.

Follow-ups

• The async contract-test service follows as a stacked PR (SDK-2868) on top of this branch.
• FDv2 support for the async client is deferred to PR 11.

Verification

• uv run pytest ldclient/testing --ignore=ldclient/testing/integrations -q → 1245 passed, 2 skipped (integration suites require Redis/DynamoDB/Consul services)
• pycodestyle, isort --check --atomic, and mypy clean on all changed files

---

Note

Overview
Adds an experimental AsyncLDClient for asyncio/aiohttp apps, wired to the FDv1 data system. Construction is loop-free; await start() / await close() (or async with) own loop-bound work like the HTTP session, data source, event processor, and big-segment polling.

Supports the usual evaluation and analytics surface (variation, variation_detail, all_flags_state, migrations, hooks, trackers). Status providers and flag_tracker are available before start(). FDv2 raises NotImplementedError for now.

Also adjusts shared plumbing so async import stays optional and components can be built without a running loop: lazy AsyncLDClient via __getattr__, get_plugin_hooks now takes a plugins list, deferred big-segment poll / flag-tracker scheduler start, plus async null stubs and parity tests.

^{Reviewed by Cursor Bugbot for commit 948902c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 4 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit bcd5bdd. Configure here.}