Refactor sub_index validation logic in LedgerEntry

[TLuca-coder210]

Overview of change

This PR fixes issue #6770 by tightening the input validation for the sub_index parameter in the ledger_entry RPC handler (LedgerEntry.cpp). It ensures that malformed negative requests return a clean validation error instead of throwing an unhandled jsoncpp exception.

Context of Change

In parseDirectory, the code used isConvertibleTo(json::ValueType::UInt) which incorrectly returns true for negative integers (like -1). When .asUInt() was called later on line 389, it threw a Json::error, causing a poor rpcINTERNAL error path for API v1 users.

This change updates the validation condition to strictly check for isUInt() (or non-negative integers), routing invalid inputs correctly to rpcINVALID_PARAMS via invalidFieldError.

API Impact

• Public API: Breaking change (Fixes error response consistency for API v1 from rpcINTERNAL to rpcINVALID_PARAMS on invalid negative bounds)
• Public API: New feature
• libxrpl change
• Peer protocol change

Before / After

• Before: Sending {"sub_index": -1} passed validation but caused an unhandled internal exception in asUInt(), returning rpcINTERNAL.
• After: Sending {"sub_index": -1} fails validation early and cleanly returns rpcINVALID_PARAMS via invalidFieldError.