[BUG] get_dsp(path) lets JSON parse errors terminate consumers

[sdatkinson]

Summary

nam::get_dsp(const std::filesystem::path&) lets JSON parser exceptions escape when a .nam file is empty or contains malformed JSON. Consumers that handle Core's normal std::runtime_error load failures can therefore terminate instead of reporting a recoverable model-load error.

This is the upstream cause of sdatkinson/NeuralAmpModelerPlugin#569.

Reproduction

On current main (3cde95c354d5ba6da01316cad90b05cfc4855053):

cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build --target loadmodel
touch /tmp/empty.nam
build/tools/loadmodel /tmp/empty.nam

Observed on macOS with AppleClang 16:

Loading model [/tmp/empty.nam]
libc++abi: terminating due to uncaught exception of type nlohmann::json_abi_v3_12_0::detail::parse_error: [json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON

The process exits with status 134.

Root cause

The path overload in NAM/get_dsp.cpp reads directly into a nlohmann::json value:

std::ifstream i(config_filename);
nlohmann::json j;
i >> j;

An empty or malformed file makes operator>> throw nlohmann::json::parse_error. That exception is not translated to the std::runtime_error used by the surrounding file/model validation paths, so it escapes consumers such as NeuralAmpModelerPlugin's staged model loader.

Current origin/main still has the unguarded parse.

Expected behavior

Loading an empty or malformed .nam file should fail through Core's recoverable model-load error path, with a useful message, rather than allowing a JSON implementation exception to escape and terminate a consumer.

Acceptance criteria

• get_dsp(path) reports empty and malformed JSON as a recoverable load failure consistent with the rest of the API.
• The error message identifies that the .nam file could not be parsed (and ideally includes the path and parser detail).
• Tests cover at least an empty .nam file and malformed JSON.

Upstream context: sdatkinson/NeuralAmpModelerPlugin#569

[sdatkinson]

Since I expect that get_dsp() is not called from a real-time thread, I am okay with exception-throwing & handling to resolve this issue so we're no longer having an unhandled exception. I'd like there to be a custom exception type thrown related to a .nam file validation failure so that it can be caught precisely.