[contract] - Add batch mint operation

[Hydrax117]

Location: contracts/ax-token/src/lib.rs

No bulk minting capability.

Acceptance Criteria:

• Mint multiple recipients in one tx
• Gas optimization for batches
• Atomic: all-or-nothing
• Event per mint
• Supply cap check

[bellofuad34-ctrl]

@bellofuad34-ctrl has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I'm familiar with this area of the codebase and ready to take this on. Can you assign it to me, i will submit PR today.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @bellofuad34-ctrl to this issue.