AccountDelete: Add explicit guard for AMM pseudo-accounts via `sfAMMID`

[mvadari]

Issue Description

In AccountDelete (src/libxrpl/tx/transactors/account/AccountDelete.cpp), AMM pseudo-accounts are protected from deletion only incidentally: the ltAMM ledger entry type is absent from the nonObligationDeleter switch (lines 175–200), so nullptr is returned, which causes tecHAS_OBLIGATIONS. This protection is a side-effect of the switch's default case, not an explicit design guard.

If ltAMM is ever added to nonObligationDeleter in a future refactor (e.g., for cleanup purposes), the protection silently disappears.

Steps to Reproduce

Not currently exploitable — AMM accounts cannot be deleted today.

Expected Result

Add an explicit check in AccountDelete::preclaim to reject deletion of AMM pseudo-accounts:

if (sleAccount->isFieldPresent(sfAMMID))
    return tecHAS_OBLIGATIONS;

This makes the intent explicit and future-proof.

Actual Result

Protection relies on ltAMM being absent from the nonObligationDeleter switch — an implicit, fragile invariant.

Environment

All versions with AMM support (XLS-30).

Supporting Files

N/A