Remove redundant WeaponTicket from witness_hero exercise

[alilloig]

Summary

• Removes the WeaponTicket struct and its two-step purchase flow (mint_weapon_ticket + transfer_weapon_ticket) from the hero module, replacing it with a single attach_weapon call that handles payment inline
• Updates both tests to use the simplified single-transaction flow
• Adds a skeleton mint_weapon function signature in the weapon module TODO for students to complete

Test plan

• sui move test passes in the solution branch (witness_hero/hero/)
• Both TODOs in the student version remain for students to implement

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