test(poller): cover independent contract polling

[CodedMumu]

Summary

• Add an integration regression test with a 2.5-second Horizon delay for one contract and a 1-second deadline for a second contract's webhook.
• Verify the fast contract progresses while the slow contract is still waiting.
• The upstream implementation already runs each contract in its own task with independent cursor state; this adds focused coverage for the reported isolation guarantee.

Tests were not run locally as requested; GitHub CI will validate the change.

Closes #6

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