Add unit tests for the escrow deposit function rejecting amounts below the minimum investment floor

[Chucks1093]

Summary

The escrow deposit function should enforce a minimum investment amount to prevent dust deposits that waste ledger entries. Unit tests should confirm deposits below the minimum panic and deposits at or above the minimum succeed.

Scope

• Test a deposit of exactly the minimum amount succeeds and is recorded in escrow state
• Test a deposit of minimum - 1 stroop panics with amount_below_minimum
• Test a deposit of 0 panics with zero_amount
• Test a deposit well above the minimum succeeds
• Assert escrow state unchanged after any panic

Acceptance Criteria

• Deposit at minimum succeeds
• Deposit below minimum panics with amount_below_minimum
• Zero deposit panics with zero_amount
• Escrow state unchanged after any panic

ETA: 12 hours

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Coordinate on Telegram