Fix singleton implementation of OmenSubgraphHandler

[coderabbitai]

The current singleton implementation of OmenSubgraphHandler appears to be buggy. The __init__ method is being called every time an instance is created, indicating that the Singleton pattern is not working as intended.

Please refer to the discussion in PR #482 Comment.

Requester: @kongzii

[gabrielfior]

Singleton working as expected.

if __name__ == "__main__":
    print("start")
    a = OmenSubgraphHandler()
    b = OmenSubgraphHandler()
    print(a, b)
    print("end")

start
a
<prediction_market_agent_tooling.markets.omen.omen_subgraph_handler.OmenSubgraphHandler object at 0x1372e0050> <prediction_market_agent_tooling.markets.omen.omen_subgraph_handler.OmenSubgraphHandler object at 0x1372e0050>
end

Closing this since not needed.