Add contract test: leave group edge cases

[WuXieSec]

This PR adds comprehensive edge case tests for the leave_group function, as requested in #29.

Changes:

• Test admin cannot leave their own group → Unauthorized error
• Test leaving during Active state fails → GroupNotForming error
• Test non-member leaving fails → NotMember error
• Test leaving updates member_groups index correctly

Test coverage:
These tests ensure leave_group properly:

• Prevents admins from abandoning their own groups
• Only allows leaving during the Forming phase
• Rejects attempts from non-members
• Maintains data consistency in the member index

Why this matters:
The leave function has several important constraints:

• Admins must stay with their groups (they can transfer admin rights first)
• Members can only leave before the group starts
• The member index must stay in sync with group membership

These tests validate all these rules are enforced correctly.

Closes #29