fix: Remove duplicate pseudo-account rejection from CredentialCreate

CredentialCreate::preclaim already rejects a pseudo-account Subject via a
separate amendment merged into develop while this branch was in review.
Drop the duplicate check added here and keep only the credential cleanup
this branch is actually for.

Adjust the Vault, LoanBroker, and AMM tests to build the pre-existing pin
with that other amendment disabled instead of this one, and drop the
now-redundant assertions that a new pin gets rejected -- that is covered
elsewhere now.

Add an overflow test for LoanBroker mirroring the existing Vault one,
exercising the tecINCOMPLETE branch a reviewer flagged as uncovered.

Fix an unrelated SeqProxy/namespace-rename mismatch in LoanBroker_test.cpp
introduced by the same develop merge, and rewrite comments across the
changed files to describe the rule rather than name the amendment or
ticket.

include/xrpl/ledger/helpers/CredentialHelpers.h

@@ -37,10 +37,10 @@ deleteSLE(ApplyView& view, SLE::ref sleCredential, beast::Journal j);
  * @brief Remove credentials pinned to a pseudo-account's owner directory.
  *
  * Cleans up credentials that were linked to a pseudo-account (Vault, LoanBroker,
- * AMM) before fixCleanup3_4_0, which such an account can neither accept nor
- * delete. Only credentials are removed, at most @p maxNodesToDelete of them; on
- * reaching that bound the result is `tecINCOMPLETE` and the caller must
- * propagate it so a later transaction resumes.
+ * AMM), which such an account can neither accept nor delete. Only credentials
+ * are removed, at most @p maxNodesToDelete of them; on reaching that bound the
+ * result is `tecINCOMPLETE` and the caller must propagate it so a later
+ * transaction resumes.
  *
  * @param view Mutable ledger view.
  * @param pseudoAcct The pseudo-account whose directory is cleaned.