SWAP-449 fix: let callers override the wrapped tx fee payer

[xianlinc]

Summary

Adds an optional fee_payer: Option<Pubkey> field to SquadsWrapConfig. When set, the given pubkey is used as the outer transaction's fee payer instead of members[0]. The override must be a multisig member or a signer pulled from the swap instructions, otherwise the build fails with InvalidConfig. When unset, behavior is identical to today.

The RFQ squads flow needs this. The maker is a signer on the inner Fill but never the outer fee payer, so the multisig member ends up paying gas and ultra-api's confirmation lookup can't match the maker's signature to the wrapped message. Once rfq-api passes fee_payer: Some(maker), the maker sits at account_keys[0] and the existing ultra-api logic works.

Linear: SWAP-449.

No breaking change

fee_payer defaults to None, which keeps members[0] as the fee payer. Existing callers see no behavioral or wire-format change. Recompiling against the new SDK requires adding fee_payer: None to existing SquadsWrapConfig { ... } literals, that's it.

Market maker validation is unaffected. is_squads_message, unwrap_transaction, and validate_fill_sanitized_message read the inner Fill instruction, not the outer fee-payer slot, so they don't care which signer sits at index 0 of the wrapped message.

PR #78's signer check operates on the unwrapped inner Fill message, not on the squads-wrapped outer message, so it doesn't collide with this change either.

Test plan

• cargo test -p squads-sdk (29/29 pass, including 2 new tests covering the override path and the invalid pubkey rejection)
• cargo clippy -p squads-sdk --tests clean
• cargo fmt --check clean
• Pair with rfq-api change to pass fee_payer: Some(maker) and re-run a mainnet RFQ swap on a Squads vault, confirm execute returns Success

[github-actions]

Coverage report

Filename	Lines Covered (%)	Functions Covered (%)
order-engine-sdk/src/fill.rs	94.92	84
order-engine-sdk/src/lib.rs	0	0
order-engine-sdk/src/transaction.rs	0	0
programs/order-engine/src/error.rs	100	100
programs/order-engine/src/instructions/fill.rs	98.29	80
programs/order-engine/src/lib.rs	90.91	75
server-example/src/config.rs	0	0
server-example/src/server.rs	0	0
squads-sdk/src/accounts.rs	93.51	100
squads-sdk/src/config.rs	39.68	50
squads-sdk/src/error.rs	0	0
squads-sdk/src/lib.rs	100	100
squads-sdk/src/pda.rs	98.04	100
squads-sdk/src/serialize.rs	70.83	60
squads-sdk/src/settings.rs	94.02	100
squads-sdk/src/transaction.rs	44.33	44.44
squads-sdk/src/unwrap.rs	89.43	71.43
squads-sdk/src/wrap.rs	84.28	85.71
webhook-api/src/enums.rs	88.75	18.18
webhook-api/src/requests.rs	0	0
webhook-api/src/responses.rs	0	0

[xianlinc]

@ioan bump for the review on this issue, once its merged in we need to switch the rfq repo to use this new version of squads SDK

[ioanSL]

LGTM 👍🏽