chore: Guard that import ldclient does not require aiohttp

[jsonbailey]

Adds a regression test asserting that a bare import ldclient does not import aiohttp.

aiohttp is an optional dependency (the async extra), so sync-only installs do not have it. The async code keeps its aiohttp-importing pieces (the async client and HTTP transport) off the eager import path: AsyncLDClient is exported lazily, and async_migrator keeps its AsyncLDClient reference behind TYPE_CHECKING. import ldclient also loads ldclient.migrations, so this covers the async migration surface too.

Nothing enforced this invariant before — a future async change could silently make import ldclient require aiohttp and break sync-only users. The test runs in a fresh interpreter so the result is not polluted by other tests that already imported aiohttp.

Tracked internally: SDK-60

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Note

Low Risk
Test-only change with no production code modifications; low risk to runtime behavior.

Overview
Adds a regression test in test_aio.py (TestImportSafety) so sync-only installs stay safe: a bare import ldclient must not load aiohttp (optional async extra).

The check runs import ldclient in a fresh subprocess and fails if aiohttp appears in sys.modules, avoiding pollution from other tests that already import async transport code in the same interpreter.

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