feat(tests): add full multisig flow test suite with scenarios for pro…

[samieazubike]

…posal lifecycle

Description

Adds a dedicated multisig-flow test suite for the group_treasury contract (closes #127). The new mod multisig_flow in contracts/contracts/group_treasury/src/test.rs covers all 10 scenarios from the issue, reusing the existing voting_setup / seed_proposal / mock_token helpers. No production contract code is changed.

4 active, passing tests (against currently-implemented APIs):

• multisig_approvals_reach_threshold_passes — approvals reaching threshold flip to Passed (scenario 3)
• multisig_double_vote_panics — a member voting twice panics (scenario 4)
• multisig_rejection_blocks_proposal — rejections reaching the blocking minority flip to Rejected (scenario 5)
• multisig_approve_after_expiry_panics — approving after expiry panics (scenario 9)

6 documented, ignored stubs carrying an explicit #[ignore = "blocked on …"] reason and the exact ledger-sequence (set_sequence_number / ttl_ledgers) assertions to enable once their dependencies land:

• multisig_propose_by_member, multisig_propose_by_non_member_panics — blocked on Implement propose_withdraw on Group Treasury contract #122 (propose_withdraw)
• multisig_execute_on_approved, multisig_execute_on_pending_panics — blocked on a group_treasury execute entrypoint (no issue yet)
• multisig_finalize_expired — blocked on an expiry finalizer (no issue yet)
• multisig_get_pending_proposals_filtering — blocked on get_pending_proposals (no issue yet)

The stub bodies call no not-yet-generated client methods, so the crate compiles cleanly.

⚠️ Note on acceptance criteria: #127's "all 10 tests pass" cannot be fully met on main today — propose_withdraw (#122), execute, finalize, and get_pending_proposals don't exist yet. The 6 stubs are ready to activate as soon as those functions merge. When #122 moves expiry from timestamp() to ledger sequence, multisig_approve_after_expiry_panics should switch set_timestamp → set_sequence_number (noted inline).

Verification

• cargo test -p group_treasury → 35 passed, 6 ignored, 0 failed
• cargo fmt -p group_treasury -- --check → clean
• cargo clippy -p group_treasury --tests → no warnings

Type of change

• Bug fix
• New feature
• Documentation update
• Other (test coverage)

Checklist

• I have read the contributing guidelines
• I have tested my changes locally
• My code follows the project's coding standards

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