Validate account/ident type in gateway_balances

[luisfernandomendozav]

High Level Overview of Change

The gateway_balances handler read the account / ident parameters with
asString() without first checking that they are strings. Passing a
non-string value (number, object, array) caused JsonCpp to throw a
Json::LogicError, which the RPC framework caught and reported to the client
as a generic rpcINTERNAL ("Internal error"). This change validates the field
type up front and returns a clean rpcINVALID_PARAMS instead.

Fixes #6758.

Context of Change

This is a defense-in-depth input-validation gap (severity: low). The bug has
existed since the handler used unchecked asString() on these fields. Before
this change, {"account": 123} or {"account": []} returned rpcINTERNAL,
which is confusing and hides the real problem (a malformed request).

The fix mirrors the existing hotwallet handling in the same file, which
already checks isString() before calling asString(), and follows the same
pattern used for other RPC handlers in the input-validation cleanup (e.g. the
peer parameter in account_lines). Non-string account/ident now return
rpcINVALID_PARAMS via RPC::invalidFieldError.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

No public methods or fields change. Only the error code for malformed
(non-string) account/ident input changes, from rpcINTERNAL to the more
accurate rpcINVALID_PARAMS.

Test Plan

Added a testGWBInvalidAccount case to GatewayBalances_test.cpp (modeled on
the existing testGWBApiVersions) that submits a non-string account and a
non-string ident and asserts the response is an invalidParams error across
all API versions. Run with:

rippled --unittest=ripple.rpc.GatewayBalances

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