Python: Add OpenFeature provider

[msiebert]

Summary

• Adds openfeature-provider/ package implementing OpenFeature provider
• Wraps local or remote flags provider, identity check for flag detection
• 29 tests, all passing

Design

• targetingKey not special, STATIC reason, three error codes, shutdown no-op

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.60638% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.87%. Comparing base (67424eb) to head (b6719df).

Files with missing lines	Patch %	Lines
...ture-provider/src/mixpanel_openfeature/provider.py	92.20%	3 Missing and 3 partials ⚠️
mixpanel/flags/local_feature_flags.py	33.33%	2 Missing ⚠️
mixpanel/flags/remote_feature_flags.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   94.20%   94.87%   +0.66%     
==========================================
  Files           9       12       +3     
  Lines        1554     1930     +376     
  Branches      100      116      +16     
==========================================
+ Hits         1464     1831     +367     
- Misses         56       62       +6     
- Partials       34       37       +3     

Flag	Coverage Δ
openfeature-provider	53.78% <97.60%> (?)

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[efahk]

Since the primary purpose of the openfeatureprovider is to not have to worry about handling the internals of the providers behind the scene, I think the init should just take in properties we need to initialize the mixpanel library and the requested flags provider

provider = MixpanelProvider(token: str, evaluation_mode: Local|Remote, ..., ..., ... ) 

[msiebert]

This is a really good point, and for a lot of other OpenFeature providers, it looks like this is how things are handled. I have a couple concerns though (kind of more in a general sense, and not specifically with Python):

1. For the client side SDKs, we need to be able to call mixpanel.identify to get all the distinctId stuff in
2. If we create a Mixpanel client within the OpenFeature provider, mixpanel.track calls on the instance outside the provider are no longer passing through the instance that the OpenFeature provider is using, which breaks runtime events' ability to detect those triggers. This will break client SDKs, and will preclude us from implementing the feature on server side SDKs, and incur the wrath of Neha.

I personally don't think it's too painful to pass in the mixpanel client, since it's just one more line, but I do understand that this deviates from the usual pattern. What do you think @efahk ?