fix(analytics): allow capability to offload reportExposure to async thread (SDK-80)

[tylerjroach]

Summary

Exposure tracking on the sync path called the user's tracker — and therefore an HTTP POST — inline, blocking every get_variant / get_variant_value / is_enabled call by the full /track round trip. The async paths already use asyncio.create_task; only the sync paths were paying the cost.

Add an optional exposure_executor: concurrent.futures.Executor | None = None field to FlagsConfig (inherited by both LocalFlagsConfig and RemoteFlagsConfig). When set, the sync providers dispatch the tracker call via executor.submit; flag evaluation returns as soon as the local logic finishes. None (the default) preserves the existing inline behavior — no breaking change for current users.

Usage

from concurrent.futures import ThreadPoolExecutor
from mixpanel.flags.types import LocalFlagsConfig

executor = ThreadPoolExecutor(max_workers=1, thread_name_prefix="mixpanel-exposure")
config = LocalFlagsConfig(exposure_executor=executor)

Context

Linear: SDK-80. Mirrors mixpanel-java#85. Audit-driven; same fix being applied to mixpanel-ruby and mixpanel-go in parallel PRs.

Test plan

• Full flags suite passes (83 tests, including the existing inline-exposure tests — exposure_executor=None keeps the old behavior unchanged)
• New test on both sync local and sync remote providers asserts the tracker runs on the executor's thread (thread_name_prefix="exposure") and not the calling thread

[codecov]

Codecov Report

❌ Patch coverage is 98.76543% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.02%. Comparing base (58d9c38) to head (f810e2f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
mixpanel/flags/local_feature_flags.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   94.50%   96.02%   +1.52%     
==========================================
  Files          12       13       +1     
  Lines        1947     3270    +1323     
  Branches      116      223     +107     
==========================================
+ Hits         1840     3140    +1300     
- Misses         70       86      +16     
- Partials       37       44       +7     

Flag	Coverage Δ
openfeature-provider	45.39% <21.73%> (-8.38%)	⬇️

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[linear-code]

SDK-80