fix: account for proposer exclusion in multisig rejections

[sirakinb]

Summary

• compute remaining possible approvals from actual eligible approvers instead of total_signers - rejections
• exclude the proposer from the approval pool while avoiding double-subtraction when the proposer rejects
• add a regression test for the 3 signer / 2 required case where one non-proposer rejection makes the tx impossible to approve

Tests

• cargo fmt -p pulsar-multisig-treasury --check
• cargo +1.91.1 test -p pulsar-multisig-treasury

Refs #556

[sirakinb]

I attempted to rebase this branch onto current main, but the conflict is because main now includes an equivalent rejection-threshold fix and regression test for non-proposer rejections. I am leaving this PR as-is for traceability rather than force-pushing a no-op branch over it. If you prefer, I can close it as superseded by the upstream fix.