Migrate multipart/form-data handling to the cpp-httplib >= 0.20 req.form API

[bburda]

Summary

The gateway's multipart/form-data handling (bulk-data and script uploads) is written against the cpp-httplib multipart API that existed up to 0.15: the httplib::Request::files map and the httplib::MultipartFormData / httplib::MultipartFormDataItems types. cpp-httplib 0.26 (shipped by Ubuntu 26.04 / ROS 2 Lyrical) removed that surface in favour of a new req.form-based API.

As a result, medkit_find_cpp_httplib (in ros2_medkit_cmake) caps the system cpp-httplib at < 0.20 and falls back to the vendored 0.14.3 header on distros that ship 0.26+. This keeps Lyrical building, but pins us to the vendored copy, which then misses upstream fixes. The pin is meant to be temporary.

---

Proposed solution

Migrate the multipart code paths to the cpp-httplib >= 0.20 req.form API, decoupling the gateway's typed MultipartBody from the cpp-httplib struct names so handlers no longer depend on the removed types. Affected sites:

• src/openapi/route_registry.hpp - multipart_upload<T>() reads req.files to populate MultipartBody.parts
• include/ros2_medkit_gateway/http/response_types.hpp - MultipartBody::parts is typed as httplib::MultipartFormDataItems
• src/http/handlers/bulkdata_handlers.cpp - consumes parts as httplib::MultipartFormData
• src/http/handlers/script_handlers.cpp - same

Once migrated, drop the < 0.20 cap in medkit_find_cpp_httplib (both the pkg-config and find_package tiers) so the system package is used on Lyrical/Resolute.

---

Additional context

Blocks using the system cpp-httplib on Ubuntu 26.04 / ROS 2 Lyrical (and any future distro shipping 0.20+). Until then the vendored 0.14.3 header is used on those distros.