fix(treasury): reject threshold updates unreachable by signer weights

[Markodiba6399]

Summary

• update_threshold now rejects any new threshold greater than the sum of all registered signer weights (ThresholdUnreachable), preventing an admin from permanently deadlocking settlement execution.
• Added TreasuryError::ThresholdUnreachable (variant 18) in crates/multisig/src/lib.rs.
• Updated admin_can_update_threshold (previously set threshold=5 with only weight=1 available, which is now correctly rejected) and added two new tests: threshold above total weight is rejected, threshold equal to total weight is accepted.

Closes #224

Validation performed

• Reviewed change by inspection; logic mirrors the existing get_all_signers weight-sum pattern used elsewhere in the contract.
• Could not run cargo test --all in this environment: main's committed Cargo.lock is out of sync with Cargo.toml (duplicate entries for autocfg/base16ct, causing a lockfile parse failure), and a full cargo generate-lockfile regeneration pulls in incompatible transitive dependency versions (ed25519-dalek vs rand_core/rand_chacha) that fail to compile even on a clean checkout of main with no changes from this PR. This is a pre-existing, unrelated repository issue — flagging it here rather than fixing it as part of this diff.

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