Validate debt type before withdrawal credit-to-debit flip

[liobrasil]

Summary

Prevent confusing "Internal error" reverts by validating debt type before a withdrawal flips a balance into debt.

Previous behavior (bug example)

Example position state:

• Existing debt type: FLOW debt
• Existing collateral type being withdrawn: MOET credit

Action:

• User withdraws MOET in a flow that would flip MOET from Credit to Debit.

Before this fix:

1. withdrawAndPull first called recordWithdrawal.
2. The position could temporarily contain two debt types (FLOW + MOET).
3. Then validateDebtType ran and hit this invariant assert:
   • "Internal error: Position has multiple debt types"

User-facing error improvement

• Before: invalid flow could fail with invariant assert
  • "Internal error: Position has multiple debt types"
• After: the same invalid flow fails with the intended domain constraint message from validateDebtType
  • "Position already has debt type X. Cannot borrow Y. Only one debt type allowed per position."

Result: transaction still reverts safely (no fund loss), but now returns a clearer and actionable error.

Why this fix is needed

PR #184 adds temporary one-debt-type guardrails. Validation should happen before mutation on flip paths (check-before-effect) so users get deterministic, domain-specific errors and we avoid relying on post-mutation invariant panics.

What changed

• In withdrawAndPull:
  • Detect when a credit withdrawal would flip to debit by comparing withdraw amount vs true credit balance before mutation.
  • Run position.validateDebtType(type) before recordWithdrawal for that flip path.
  • Remove the post-mutation flip validation block.

Tests

• Ran:
  • flow test cadence/tests/single_token_constraint_test.cdc
  • flow test cadence/tests/debt_type_constraint_three_token_test.cdc