test(energy-token): validate SEP-41 metadata and trustline edge cases

[katecatherina]

Closes #560

Summary

Adds comprehensive SEP-41 compliance tests for the energy_token contract covering metadata correctness and trustline edge cases:

Metadata validation

• test_sep41_name_is_correct — name returns "SolarProof kWh"
• test_sep41_symbol_is_correct — symbol returns "SKWH"
• test_sep41_decimals_is_three — decimals returns 3

Trustline / default-state edge cases

• test_sep41_zero_balance_unknown_account — uninitialized address has balance 0 (SEP-41 implicit trustline)
• test_sep41_allowance_defaults_to_zero — no prior approval → allowance is 0

Invalid-amount edge cases

• test_sep41_approve_negative_amount_panics
• test_sep41_transfer_zero_amount_panics
• test_sep41_burn_zero_amount_panics
• test_sep41_transfer_from_zero_amount_panics
• test_sep41_burn_from_zero_amount_panics

Behavioral edge cases

• test_sep41_transfer_to_self — balance unchanged after self-transfer
• test_sep41_allowance_not_shared_between_spenders — independent allowances per spender
• test_sep41_total_supply_after_multiple_mints — supply accumulates correctly across recipients
• test_sep41_decimals_conversion_1kwh — 1000 units = 1 kWh at 3 decimals

Testing

cd apps/contracts && cargo test -p energy-token

Checklist

• Tests added to energy_token/src/lib.rs
• No production code changed

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