[CONTRACTS] Implement bounty claim function

[DavisVT]

Implement the claim_bounty function that releases locked USDC to the contributor after maintainer approval.

[TUPM96]

/attempt #2

Plan: implement maintainer-approved claim_bounty for the Soroban bounty contract, release the locked token escrow to the contributor only after maintainer auth, clear the stored bounty after claim to prevent double payout, and add token-backed tests verifying maintainer balance decreases on create and contributor/contract balances update on claim. Payout details can be provided privately if needed.

[liangtovi-debug]

/attempt #2

Implementation plan:

1. Add claim_bounty(token, maintainer, contributor, issue_id) in the Soroban contract and require maintainer auth for approval.
2. Load escrowed amount from persistent storage key (bounty, issue_id) + maintainer mapping and fail closed when missing.
3. Transfer the locked token amount from contract escrow to contributor, then clear storage entry to prevent double payout.
4. Add token-backed tests validating maintainer -> contract transfer on create, contract -> contributor transfer on claim, and panic on second claim.
5. Submit PR with cargo test evidence.

[liangtovi-debug]

/claim

Implemented in PR:

[liangtovi-debug]

/claim

Implemented in PR: #10

What changed:

• Added claim_bounty(env, token, maintainer, contributor, issue_id) to release escrowed tokens to the contributor after maintainer approval.
• Reads escrow amount from (bounty, issue_id) persistent storage and panics if no matching maintainer bounty exists.
• Clears the maintainer entry after payout to prevent double claims.
• Added token-backed tests for successful claim payout and double-claim prevention.

Test evidence:

• cd contracts/bounty && cargo test
• Result: 2 passed, 0 failed.