feat: new parseConfiguration for error feedback

[greghuels]

Stacked onto #35

Summary

This PR introduces a new parseConfiguration() convenience function that provides comprehensive error feedback when parsing flag configurations and bandit models. The new API replaces the previous from_json pattern with explicit error tracking, making it easier to diagnose configuration issues.

What Changed

New Public API

Added parseConfiguration() convenience function with two overloads:

// Parse flags only
Configuration parseConfiguration(const std::string& flagConfigJson,
                                 std::string& error);

// Parse flags and bandit models together
Configuration parseConfiguration(const std::string& flagConfigJson,
                                 const std::string& banditModelsJson,
                                 std::string& error);

Key Features

• Returns Configuration directly (no intermediate types needed)
• Error messages via reference parameter with detailed diagnostics
• Consolidates all parsing errors into a single formatted message
• Optional bandit models support (pass empty string or use two-parameter overload)

Breaking Changes

1. Replaced from_json with internal::parseBanditResponse
   - Old: BanditResponse response = json_object;
   - New: BanditResponse response = internal::parseBanditResponse(jsonStr, error);
   - Provides error tracking and better diagnostics
2. Moved parsing functions to internal namespace
   - parseConfigResponse() → internal::parseConfigResponse()
   - parseBanditResponse() → internal::parseBanditResponse()
   - These are now implementation details; use parseConfiguration() instead

Migration Guide

Before:

 nlohmann::json flagsJson = nlohmann::json::parse(flagConfigJson);
 ConfigResponse flagConfig = flagsJson;

 nlohmann::json banditsJson = nlohmann::json::parse(banditModelsJson);
 BanditResponse banditModels = banditsJson;

After:

  std::string error;
  Configuration config = parseConfiguration(flagConfigJson, banditModelsJson, error);
  if (!error.empty()) {
      std::cerr << "Configuration error: " << error << std::endl;
      return 1;
  }

  Flags Only:
  std::string error;
  Configuration config = parseConfiguration(flagConfigJson, error);
  if (!error.empty()) {
      std::cerr << "Configuration error: " << error << std::endl;
      return 1;
  }

[greghuels]

Closing since this is mostly captured in #40. I'll open a new PR for the parseConfiguration function