fix: Issue 6787 catch all std::exception in getAsset (AMMInfo)

[shortthefomo]

High Level Overview of Change

Fix getAsset in the amm_info RPC handler catching only std::runtime_error from assetFromJson/issueFromJson, leaving Json::error and other std::exception subtypes uncaught. Such exceptions now propagate to the top-level callMethod handler and return an opaque rpcINTERNAL instead of the expected rpcISSUE_MALFORMED error.

Fixes #6787

Context of Change

Bug — present since getAsset (formerly getIssue) was introduced in AMMInfo.cpp.

assetFromJson delegates to issueFromJson, which throws Json::error (a subclass of std::exception, not std::runtime_error) for invalid currency or issuer string values — for example:

// issueFromJson throws Json::error here, not std::runtime_error:
Throw<Json::error>("issueFromJson currency must be a valid currency");
Throw<Json::error>("issueFromJson issuer must be a string Json value");

The original catch clause catch (std::runtime_error const& ex) did not cover these paths. Any Json::error thrown by issueFromJson would escape getAsset uncaught, propagate up through the RPC dispatch, be caught by the generic callMethod handler, and return rpcINTERNAL — a confusing error that gives the client no indication of the actual problem (a malformed asset field).

Fix: Widen the catch to catch (std::exception const& ex) so that all exception types thrown by assetFromJson/issueFromJson are handled locally and mapped to rpcISSUE_MALFORMED.

Before / After

Before — client sends amm_info with {"asset": {"currency": "!!!BAD!!!"}}:

{
  "result": {
    "error": "internal",
    "error_code": 73,
    "error_message": "Internal error.",
    "status": "error"
  }
}

After — same request returns the descriptive malformed-issue error:

{
  "result": {
    "error": "issueMalformed",
    "error_code": 46,
    "error_message": "Issue is malformed.",
    "status": "error"
  }
}

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 API impact. The change only affects the error-path for malformed asset/asset2 inputs to amm_info; all well-formed requests are unaffected.

Test Plan

Added testGetAssetExceptionCoverage to AMMInfo_test.cpp. It sends an amm_info request with an asset field containing an invalid currency string ("!!!INVALID!!!") — the path that previously triggered Json::error and returned rpcINTERNAL. The test asserts:

1. The response contains error (the request was correctly rejected).
2. The error is not "internal" — confirming the exception is now caught locally and mapped to the correct error code rather than escaping to the generic handler.