Fossil client callback settles round

[eugenPtr]

Changes Summary

These changes consolidate the settlement process into the fossil callback, removing the need for a separate settlement step and ensuring atomic updates of L1 data and round settlement.

Core Changes

• Modified fossil_callback in FossilClient to return a FossilCallbackReturn enum instead of void
• Removed separate settle_round function from Vault contract - settlement now happens directly in fossil_client_callback
• Added new return types:
  • FossilCallbackReturn enum with variants:
    • RoundSettled(RoundSettledReturn)
    • FirstRoundInitialized
  • RoundSettledReturn struct containing total_payout

Contract Changes

Vault Contract

• Modified fossil_client_callback to:
  • Handle settlement directly
  • Return the appropriate FossilCallbackReturn variant
  • Not store L1 pricing data anymore
  • Trigger settlement in same transaction

FossilClient Contract

• Updated interface to include return type
• Modified callback implementation to propagate return value from vault

Test Changes

• Removed timeskip_and_settle_round helper as settlement now happens in callback
• Removed settle_option_round and related methods from VaultFacade
• Updated test assertions to handle new return values
• Extracted helper methods from FossilClientFacade into a separate file -> fossil_client_helpers.cairo
• Modified test cases to work with combined callback/settlement flow