server: (router) Forward form-data to model server (Fixes #22044) (#22118)

* This commit enables the router to forward form-data to model server.
Fixes #22044 (enabling to use the /v1/audio/transcriptions in router mode)

* * Applied the suggestion from Copilots first comment: using the non-throwing json::parse overload.
* Addressed Copilots third comment by extending the files representation to also include filename and content-type
* Addressed Copilots fourth comment by making the RNG thread_local

* Changed variable body from std::string to std::ostringstream in build_multipart_body
as suggested by ngxson in https://github.com/ggml-org/llama.cpp/pull/22118#discussion_r3127099053

* Added sanitize_field lambda in build_multipart_body for key, filename and content_type
as suggested by ngxson in https://github.com/ggml-org/llama.cpp/pull/22118#discussion_r3127104647

* explicitly checking if value/item is string before calling value/item.get<std::string>()
as requested by ngxson in https://github.com/ggml-org/llama.cpp/pull/22118#discussion_r3127111279

* Added double quote to the sanitize lambda and throw on json parse failure

---------

Co-authored-by: Ralph Paßgang <ralph@trust-it.de>

tools/server/server-chat.cpp

@@ -575,14 +575,14 @@ json server_chat_msg_diff_to_json_oaicompat(const common_chat_msg_diff & diff) {
 json convert_transcriptions_to_chatcmpl(
         const json & inp_body,
         const common_chat_templates * tmpls,
-        const std::map<std::string, raw_buffer> & in_files,
+        const std::map<std::string, uploaded_file> & in_files,
         std::vector<raw_buffer> & out_files) {
     // TODO @ngxson : this function may need to be improved in the future
     // handle input files
     out_files.clear();
     auto it = in_files.find("file");
     if (it != in_files.end()) {
-        out_files.push_back(it->second);
+        out_files.push_back(it->second.data);
     } else {
         throw std::invalid_argument("No input file found for transcription");
     }