[contracts] Add WinningsClaimed event emission in claim_winnings()

[Ehonrie]

Add WinningsClaimed event to claim_winnings() in contracts/market/src/lib.rs.

Event must include: bet_id, bettor, payout, fee_paid, claimed_at.

Acceptance Criteria:

• Event emitted after every successful claim
• All fields correct including fee amount
• Unit test asserts event values

[lettalkdata-cloud]

@lettalkdata-cloud has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I will love to work on this issue

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[drips-wave]

Congratulations, @lettalkdata-cloud! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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