Add contract test: group full scenario

[WuXieSec]

This PR adds tests for the group full scenario, as requested in #27.

Changes:

• Test group with max_members=2
• Fill with 2 members
• Attempt to add 3rd member
• Verify GroupFull error
• Add additional tests for at-capacity and not-full scenarios

Why this matters:
These tests ensure:

• The max_members limit is properly enforced
• The correct error is returned when attempting to join a full group
• Groups can reach capacity without issues
• Groups with available slots accept new members

Test coverage:

• ❌ Join full group (2/2 members) → GroupFull
• ✅ Fill group to exact capacity (3/3 members)
• ✅ Join group with available slots (3/5 members)

Closes #27