fix(multisig): gate add_admin/remove_admin/update_required_signatures through execute_action

[BWM0223]

Summary

Fixes #68 - Public add_admin/remove_admin/update_required_signatures bypass multi-sig entirely

Root Cause

Three public entrypoints perform no auth check. Any address can remove admins or reduce signature threshold to 1. add_admin self-authorizes the new admin.

Fix

• Public entrypoints add_admin/remove_admin/update_required_signatures now return Unauthorized immediately
• Only execute_action() (which requires approval_count >= required_signatures) calls the _internal variants
• No path allows changing admin set or threshold without required approvals

[BWM0223]

This PR closes the auth bypass where add_admin, remove_admin, and update_required_signatures were exposed as public entrypoints with no auth checks, allowing any address to alter the admin set or reduce the signature threshold. The fix ensures these operations only execute through the multi-sig execute_action flow which requires the configured number of distinct admin approvals.