Add test for recurring disbursement failure without mutating schedule…

[akprinciple]

closes #448

Guard recurring disbursements against insufficient balance before state mutation

Summary

This PR ensures recurring disbursements fail early with the existing TransferFailed error variant before mutating any schedule state when the contract treasury lacks sufficient token balance to cover the scheduled disbursement period.

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Changes

• Pre-Execution Balance Check: Added an upfront contract balance verification in disburse_recurring prior to initiating the token transfer.
• Early Exit on Low Funds: Returns Error::TransferFailed immediately if the contract treasury balance is lower than the scheduled amount.
• State Mutation Guard: Guarantees that no recurring schedule state fields are updated on insufficient balance failures.
• Error Variant Reuse: Reused the existing TransferFailed error variant defined in the contract error enum.

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Why

Previously, the recurring disbursement flow could progress toward the transfer operation without validating available treasury liquidity. In low-balance scenarios, the transaction could abort after entering a state-mutation path, risking schedule state corruption and inconsistent accounting.

This pre-check ensures strict check-effects-interactions safety and protects schedule state integrity.

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Validation

Added a targeted regression test verifying behavior under insufficient contract balance:

• Asserts Error::TransferFailed is returned deterministically.
• Asserts last_disbursed_at, total_disbursed, and periods_disbursed remain completely unchanged.
• Asserts the recurring schedule remains in an active state.

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