Fix get_feature_flag_payload return type annotation

Update return type from Optional[str] to Optional[FeatureFlagResult] to match the actual return value from the underlying implementation.

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Co-Authored-By: Claude <[email protected]>

Author: gustavohstrassburger

posthog/__init__.py

@@ -581,7 +581,7 @@ def get_feature_flag_payload(
     only_evaluate_locally=False,
     send_feature_flag_events=True,
     disable_geoip=None,  # type: Optional[bool]
-) -> Optional[str]:
+) -> Optional[FeatureFlagResult]:
     return _proxy(
         "get_feature_flag_payload",
         key=key,