chore: Add async support classes for concurrency and HTTP transport

[jsonbailey]

What

Adds the async support classes the async SDK is built on, under ldclient/impl/aio/. Pure-additive: the package is dormant until its first consumer lands in a later slice, so it adds no runtime surface to main beyond the new (as-yet-unimported) modules.

• aio/concurrency.py — async concurrency primitives: AsyncEvent, AsyncLock, AsyncQueue, AsyncRepeatingTask, AsyncWorkerPool, AsyncTaskRunner, AsyncCallbackScheduler, and spawn_handle/join_handle. Each wraps a piece of fiddly asyncio plumbing (timeout-aware waits, queue exception normalization, an interval-from-start repeating task, a bounded task pool) that async callers would otherwise inline repeatedly.
• aio/transport.py — async HTTP/SSE transport (AsyncHTTPTransport, AsyncSSEFactory, make_client_session) over aiohttp.
• aio/transport_types.py — the shared TransportResponse.
• testing/test_aio.py — unit tests for the above (they previously shipped untested).
• pyproject.toml — the async packaging these need: the [async] aiohttp extra plus aiohttp / pytest-asyncio dev deps and asyncio_mode="strict" (the tests run under pytest-asyncio, so CI can't pass without it).

Why

Foundation for the async Python SDK (epic SDK-60). These are genuine async support classes, not a compatibility shim: there is no sync twin — the sync code uses the equivalent stdlib/SDK primitives (threading, queue.Queue, RepeatingTask, FixedThreadPool, urllib3) directly.

Consumed by later slices: concurrency.py first by async big segments and the async flag tracker, then the event processor, data sources, and data systems; transport.py first by async FDv1 streaming/polling, then the event processor and FDv2 sources.

Validation

• uv run pytest ldclient/testing/test_aio.py — 28 passed
• uv run mypy ldclient — clean
• isort / pycodestyle — clean

Refs SDK-2601 (epic SDK-60).

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Note

Low Risk
Purely additive, unimported by the main SDK at merge; risk is limited to optional dependency and test/CI configuration, not production sync behavior.

Overview
Introduces ldclient/impl/aio/ as additive groundwork for the async Python SDK: concurrency helpers and aiohttp-based networking, plus tests and packaging so CI can run them. Nothing in the shipped client imports these modules yet.

Concurrency (concurrency.py) centralizes asyncio patterns that mirror existing sync primitives (RepeatingTask, FixedThreadPool, threading-style events/queues): timeout-friendly AsyncEvent/AsyncQueue, AsyncRepeatingTask, bounded AsyncWorkerPool, AsyncTaskRunner, thread-safe AsyncCallbackScheduler, and spawn_handle/join_handle with logged/cancelled background work.

Transport (transport.py, transport_types.py) adds TransportResponse, make_client_session (TLS/certs from HTTPConfig), AsyncHTTPTransport, and AsyncSSEFactory wired to ld_eventsource with the same proxy rules as sync (_get_proxy_url, trust_env=False).

Tests & deps: ldclient/testing/test_aio.py covers primitives and HTTP/proxy behavior; pyproject.toml adds optional [async] aiohttp extra, dev aiohttp/pytest-asyncio, and asyncio_mode = "strict".

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