refactor(futarchy): replace vault transaction account with transaction_index arg

[pileks]

Summary

Replaces the vault_transaction account in admin_approve_multisig_proposal with a transaction_index argument, enabling approval of Squads multisig proposals via batch transactions.

Changes

• admin_approve_multisig_proposal.rs: Removed the vault_transaction account from the AdminApproveMultisigProposal accounts struct. Instead, the instruction now accepts AdminApproveMultisigProposalArgs containing a transaction_index: u64. The squads_multisig_proposal PDA is now derived using this index directly, rather than reading it from a passed-in vault transaction account.
• lib.rs: Updated the instruction signature to accept and forward the new args parameter, with access_control validation wired through validate(&args).
• SDK & Tests: Regenerated SDK types and updated tests to pass transactionIndex instead of the vaultTransaction account.

Motivation

Previously, approving a multisig proposal required passing the full vault_transaction account, which prevented batching multiple approve calls in a single Solana transaction (since each would reference a different vault transaction account that needed to be resolved). By taking the transaction index as a simple u64 argument and deriving the proposal PDA on-chain, callers can now batch multiple approvals without needing to supply additional accounts.

[pileks]

Despite us likely not needing it immediately, I still think we should add this. It is a minor change that allows us to run batch txns when we need them.

cc @R-K-H lmk if you agree