fix: use error_handler_t::replace for UTF-8 in JSON dump

Prevents json::type_error.316 when payload strings contain invalid
UTF-8 bytes. Replaces bad bytes with U+FFFD instead of throwing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Sunrisepeak

src/providers/anthropic.cppm

@@ -456,7 +456,7 @@ private:
         tinyhttps::HttpRequest req;
         req.method = tinyhttps::Method::POST;
         req.url = config_.baseUrl + std::string(endpoint);
-        req.body = payload.dump();
+        req.body = payload.dump(-1, ' ', false, Json::error_handler_t::replace);
 
         req.headers["Content-Type"] = "application/json";
         req.headers["x-api-key"] = config_.apiKey;

src/providers/openai.cppm

@@ -453,7 +453,7 @@ private:
         tinyhttps::HttpRequest req;
         req.method = tinyhttps::Method::POST;
         req.url = config_.baseUrl + std::string(endpoint);
-        req.body = payload.dump();
+        req.body = payload.dump(-1, ' ', false, Json::error_handler_t::replace);
 
         req.headers["Content-Type"] = "application/json";
         req.headers["Authorization"] = "Bearer " + config_.apiKey;