Refactor security_advisories tools to use NewTool pattern

Convert 4 functions from NewServerToolLegacy wrapper to NewTool:
- ListGlobalSecurityAdvisories
- GetGlobalSecurityAdvisory
- ListRepositorySecurityAdvisories
- ListOrgRepositorySecurityAdvisories

Update tools.go toolset registration and tests.

Co-authored-by: Adam Holt <omgitsads@users.noreply.github.com>

Author: SamMorrowDrums