Typed response models with compile-time schema generation

[bburda]

Problem

OpenAPI schemas are defined separately from handler response construction. There is no compile-time or runtime enforcement that a handler's JSON output matches its declared schema. This leads to silent drift - schemas say one thing, handlers return another, and generated clients break.

PR #313 fixed all current mismatches and added a SchemaConsistencyTest safety net, but the root cause remains: schemas and serialization are two independent code paths.

Proposed solution

Introduce typed C++ response model structs that own both serialization and schema generation:

struct ConfigurationMetaData {
  std::string id, name, type;

  nlohmann::json to_json() const {
    return {{"id", id}, {"name", name}, {"type", type}};
  }

  static nlohmann::json openapi_schema() {
    return {{"type", "object"},
            {"properties",
             {{"id", {{"type", "string"}}},
              {"name", {{"type", "string"}}},
              {"type", {{"type", "string"}}}}},
            {"required", {"id", "name", "type"}}};
  }
};

Handlers construct the struct, not raw JSON. Route registration extracts the schema from the type. Single source of truth - if the struct changes, both the response and the schema change together.

Scope

• Define response model structs for each resource type (Configuration, Operation, Fault, Data, Trigger, Subscription, Lock, Script, BulkData, Update, Health, Auth)
• Migrate handlers incrementally (one resource type per PR)
• Register schemas via model types instead of manual component_schemas() entries
• Remove SchemaBuilder static schema methods as models take over

Alternatives considered

• Shared field constants (struct with static constexpr field names used by both schema and handler) - less invasive but still allows structural divergence
• Runtime validation (validate response JSON against schema before sending) - catches mismatches but only at test/runtime, adds overhead

Context

• PR fix: align OpenAPI schemas with actual handler responses #313 (fix: align OpenAPI schemas with actual handler responses)
• Issue [BUG] OpenAPI spec: ConfigurationParam used for both list and detail with value required #306 (original bug report)