closes #51: re-implement batch minting feature

[mansur-codes]

Summary

This PR successfully re-implements and restores the batch_mint functionality and its accompanying comprehensive test suite in the token contract, which was previously lost during concurrent upstream branch merges.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
• 🔧 Smart contract improvement
• 🧪 Test coverage improvement
• 🏗️ CI/Build improvement

Related Issue

Closes #51

Changes Made

• Imports: Restored Recipient and TokenError imports from the parent crate module back into contracts/token/src/test.rs.
• Core Functionality: Ensured the batch minting mechanics cleanly accept multiple recipients, update individual balances, and accurately aggregate and update the global token total_supply.
• Test Suite Appended: Added 4 comprehensive unit tests:
  • test_batch_mint_single_recipient: Verifies simple batch minting state updates for one user.
  • test_batch_mint_multiple_recipients: Validates correct execution, tracking, and overall supply updates across three distinct addresses.
  • test_batch_mint_rejects_invalid_amount: Uses try_batch_mint to assert that zero/negative amounts yield a TokenError::InvalidAmount error and trigger an atomic revert.
  • test_batch_mint_while_paused_fails: Verifies that attempting execution while the contract is paused correctly returns TokenError::ContractPaused without changing state.

Testing

How has this been tested?

• Rust unit tests pass (cargo test)
• SDK compiles (npm run build in sdk/)
• Manual testing against Soroban testnet
• New tests added for changes

Test commands run:

# Contract tests
cargo test

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[mansur-codes]

Hi there, please review this issue and release points, thank you.