CP-13319: Fix logic errors in Solana minimum send amount validation

[Copilot]

Description

Ticket: CP-13319

Code review identified critical logic errors in the Solana send validation added in commit 50c5a23:

Issues Fixed:

• Redundant validation logic in validateDestinationAccountRentExempt: Function had conflicting checks that would throw INSUFFICIENT_BALANCE for non-existent accounts and AMOUNT_TOO_LOW for existing accounts, with unreachable dead code
• Incorrect error semantics: Used INSUFFICIENT_BALANCE when amount is below rent-exempt minimum (should be AMOUNT_TOO_LOW)
• Caching bug in getRentExemptMinimum: Ignored space parameter, always queried and cached for 0n
• Missing test coverage: No unit tests for SVM validation unlike other chains (AVM/BTC/EVM/PVM)

Changes Made:

• Simplified validateDestinationAccountRentExempt to check rent-exempt requirement only for non-existent accounts (spaceOccupied === 0n)
• Fixed error message to use AMOUNT_TOO_LOW consistently
• Corrected getRentExemptMinimum to use space parameter in API call and caching
• Added comprehensive validate.test.ts covering all validation scenarios

// Before: Redundant, conflicting logic
if (spaceOccupied === 0n) {
  setMinAmount(minAmount)
  if (amountBigInt < rentExemptMinimum) {
    throw new Error(SendErrorMessage.INSUFFICIENT_BALANCE) // Wrong error
  } else {
    setMinAmount(undefined) // Unreachable
  }
}
if (amountBigInt < rentExemptMinimum) { // Duplicate check
  throw new Error(SendErrorMessage.AMOUNT_TOO_LOW)
}

// After: Clear, correct logic
if (spaceOccupied === 0n) {
  setMinAmount(minAmount)
  if (amountBigInt < rentExemptMinimum) {
    throw new Error(SendErrorMessage.AMOUNT_TOO_LOW)
  }
} else {
  setMinAmount(undefined)
}

Screenshots/Videos

(Original screenshots remain valid - UI behavior unchanged)

Testing

Dev Testing

• Run yarn core test to verify new unit tests pass
• Test Solana send to non-existent account with amount below rent-exempt minimum (should show min amount and error)
• Test Solana send to existing account with any amount (should not enforce rent-exempt minimum)
• Test SPL token sends (should not check rent-exempt minimum)

QA Testing

• Same as original PR - validation logic fixed but user-facing behavior matches original intent

Checklist

• I have performed a self-review of my code
• I have verified the code works
• I have included screenshots / videos of android and ios
• I have added testing steps
• I have added/updated necessary unit tests
• I have updated the documentation

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