test(invoice-token): add test suite for mint, mint_batch, and TransferLocked error paths

[SMOGZ6783]

Summary

Adds test coverage in contracts/invoice-token/src/test.rs for error paths that were not previously exercised: mint, mint_batch, and an explicit assertion on the TransferLocked error variant.

Closes #169

New tests added

• test_mint_unauthorized_fails — non-admin/non-minter caller rejected
• test_mint_zero_amount_rejected / test_mint_negative_amount_rejected — InvalidAmount
• test_mint_while_paused_fails — Paused
• test_mint_by_admin_succeeds — admin can mint (not just the designated minter)
• test_mint_overflow_rejected — Overflow on balance overflow
• test_mint_batch_length_mismatch_rejected — BatchLengthMismatch
• test_mint_batch_unauthorized_fails — Unauthorized
• test_mint_batch_while_paused_fails — Paused
• test_mint_batch_succeeds_and_preserves_order — state persistence across batch mint
• test_transfer_fails_with_transfer_locked_error — explicit TransferLocked assertion

Testing

• cargo fmt clean
• cargo clippy --all-targets -- -D warnings clean
• cargo test --all passes, including all pre-existing tests
• New tests assert both the returned Error variant and resulting state (balance/total_supply) persistence

Acceptance criteria

• ✅ All implementation tasks completed
• ✅ Compiles without warnings or errors
• ✅ All existing tests continue to pass
• ✅ New unit tests cover previously untested error/edge paths
• ✅ PR targets dev
• ✅ cargo fmt and cargo clippy clean

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