Refactors header parsing logic

Improves header parsing by using a dedicated parsing function,
resulting in cleaner and more robust code.

This change also adds error handling during header parsing,
returning an error and marking the request as invalid
if parsing fails.

Author: kwach

httplib.h

@@ -5046,12 +5046,14 @@ class MultipartFormDataParser {
             return false;
           }
 
-          // split header string by ':' and emplace space trimmed into headers map
-          auto colon_pos = header.find(':');
-          if (colon_pos != std::string::npos) {
-            auto key = trim_copy(header.substr(0, colon_pos));
-            auto val = trim_copy(header.substr(colon_pos + 1));
-            file_.headers.emplace(key, val);
+          // parse and emplace space trimmed headers into a map
+          if (!parse_header(
+                 header.data(), header.data() + header.size(),
+                 [&](const std::string &key, const std::string &val) {
+                   file_.headers.emplace(key, val);
+                 })) {
+            is_valid_ = false;
+            return false;
           }
 
           constexpr const char header_content_type[] = "Content-Type:";