test(energy_token): add overflow/underflow test coverage for token arithmetic

[devnWisdom]

Summary

Adds a dedicated overflow_tests module with 16 tests covering every i128 arithmetic path in energy_token for overflow and underflow conditions.

Tests added

Overflow (addition wraps past i128::MAX)

Test	Path
mint_overflows_recipient_balance	balance + amount in mint()
mint_overflows_total_minted	total_minted + amount in mint()
transfer_overflows_recipient_balance	tb + amount in move_balance()
transfer_from_overflows_recipient	same path via transfer_from()

Underflow / insufficient funds (subtraction below zero)

Test	Path
burn_underflows_when_amount_exceeds_balance	deduct_balance()
burn_underflows_on_zero_balance	deduct_balance() with no prior mint
burn_from_underflows_when_balance_exhausted	deduct_balance() via burn_from()
burn_from_underflows_allowance	spend_allowance()
transfer_underflows_sender	move_balance() sender side

Zero/negative input guards

Test	Guard
mint_zero_amount_rejected	amount must be positive
mint_negative_amount_rejected	amount must be positive
burn_zero_amount_rejected	amount must be positive
transfer_zero_amount_rejected	amount must be positive
approve_negative_amount_rejected	amount must be non-negative

Closes AnnabelJoe#559

[github-actions]

🔍 Vercel Preview Deployment

URL: Learn More: https://err.sh/vercel/missing-token-value

Uses Stellar testnet contract addresses.