feat: Add async migration support

[jsonbailey]

Adds the async migration surface for the async Python SDK — AsyncMigrator, AsyncMigratorBuilder, AsyncMigrationConfig, and the AsyncMigratorFn type — in ldclient/migrations/async_migrator.py, eagerly exported from ldclient/migrations/__init__.py (import-cheap: asyncio stdlib only, no aiohttp).

Also carries the migration config/builder dedup shared by the sync and async surfaces: types.py gains _MigrationConfigBase[_MigratorFnT] (Generic) and _MigratorBuilderBase (fluent-setter mixin), and migrator.py's MigratorBuilder is re-based onto that mixin. These are behavior-preserving refactors of already-released sync code.

The public async classes carry experimental .. caution:: blocks.

Self-contained: depends only on already-merged foundation work. The AsyncLDClient/Context references are TYPE_CHECKING-only (annotations kept lazy via from __future__ import annotations); no runtime import of any not-yet-merged async module.

No CHANGELOG or version changes (handled at release).

Tracked internally: SDK-2767

---

Note

Medium Risk
New experimental migration execution path affects dual-origin reads/writes and telemetry during flag-driven cutovers; behavior is well-tested but not production-stable per API warnings.

Overview
Adds an experimental async migration API for the async Python SDK, mirroring the sync migrator’s stage-driven read/write behavior with coroutine-based old/new handlers.

New surface in async_migrator.py: AsyncMigrator / AsyncMigratorImpl, AsyncMigratorBuilder, AsyncMigrationConfig, AsyncMigratorFn, and AsyncExecutor. Reads honor ExecutionOrder (parallel, serial, random) via asyncio.gather and optional consistency checks; writes run authoritative-first and skip the second origin on authoritative failure. migration_variation is awaited; track_migration_op stays synchronous. Write cancellation logs a warning, still emits migration telemetry in finally, and propagates CancelledError without undoing a completed authoritative write.

ldclient/migrations/__init__.py re-exports the new types. test_async_migrator.py adds broad pytest coverage (builder validation, payloads, tracking, consistency, execution order, cancellation) using a FakeAsyncClient.

^{Reviewed by Cursor Bugbot for commit d35a24c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jsonbailey]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit c735e0e. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{❌ 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 ef02557. Configure here.}