feat(feature-flags): support quota limiting for feature flags #403

[dmarticus]

with PostHog/posthog#28564, we now start to respond different with the /decide and /local_evaluation APIs if users have gone over their quota limit. Now we need to change the SDKs to handle these new responses.

[greptile-apps]

PR Summary

This PR adds support for handling quota limits in PostHog's .NET SDK, responding to changes in the /decide and /local_evaluation APIs when users exceed their quota limits.

• Added QuotaLimited property to DecideApiResult in /src/PostHog/Api/DecideApiResult.cs to track which features are quota-limited
• Implemented quota limit handling in /src/PostHog/PostHogClient.cs to return empty dictionaries for bulk flag requests when limits are hit
• Added quota limit checks in DecideAsync() method to handle feature flag quota limitations
• Added debug logging for quota exceeded scenarios with appropriate messages
• Added comprehensive test coverage in /tests/UnitTests/Features/FeatureFlagsTests.cs for quota limit scenarios

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[haacked]

Thanks @dmarticus! I'll fix this up.

[dmarticus]

@haacked thanks! I was working on it locally but my .NET LSP isn't playing nicely so it's been a pain. Thanks for helping, lmk if you have questions about the implementation!

[haacked]

When attempting a local evaluation API call that is quota limited, I assume we don't want to fallback to /decide correct? If one is quota limited, do we assume the other is too? Or should we continue to fallback to /decide?

[haacked]

@dmarticus I also noticed the log level for quota exceeded is Debug in this PR. I'm thinking this is something a consumer would want to notice. It should probably be Warning or Error. Right?

[dmarticus]

When attempting a local evaluation API call that is quota limited, I assume we don't want to fallback to /decide correct? If one is quota limited, do we assume the other is too? Or should we continue to fallback to /decide?

Yup, we should short-circuit if we get a local eval call that's quota limited

[dmarticus]

@dmarticus I also noticed the log level for quota exceeded is Debug in this PR. I'm thinking this is something a consumer would want to notice. It should probably be Warning or Error. Right?

Yeah, let's go with a warning log, that's what the other PRs do.

[haacked]

Nice work!