fix: Enrich captured events with locally evaluated flags

[Josh-IE]

Problem

PostHog/posthog#31373

Changes

When capturing an event and send_feature_flags=True, use locally evaluated feature flag data, before falling back to the /flags or /decide

• if client.feature_flags is set, the feature flags on the client is evaluated and sent with the event, regardless of the send_feature_flags setting
  • if the local evaluation fails, the capture event is sent without feature flags
• if send_feature_flags=True, feature flags data is retrieved from /api/feature_flag/local_evaluation and evaluated locally, and sent with the event
  • if the local evaluation fails, it falls back to the /flags or /decide endpoint
• if personal_api_key is not set, local evaluation is disabled and instead the /flags or /decide endpoint is called

[greptile-apps]

PR Summary

This PR adds functionality to enrich captured events with locally evaluated feature flags, improving performance by reducing API calls when possible.

• Added local feature flag evaluation in capture() method to avoid unnecessary API calls when flags are already loaded
• Implemented fallback behavior to /flags or /decide endpoints when local evaluation fails
• Added comprehensive test coverage in test_client.py for local flag evaluation scenarios
• Ensured feature flag data is included in event properties regardless of send_feature_flags setting when local flags exist
• Added proper error handling for cases where personal_api_key is not set, defaulting to API calls

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