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-http.h

@@ -36,13 +36,19 @@ struct server_http_res {
 using server_http_res_ptr = std::unique_ptr<server_http_res>;
 using raw_buffer = std::vector<uint8_t>;
 
+struct uploaded_file {
+    raw_buffer data;
+    std::string filename;
+    std::string content_type;
+};
+
 struct server_http_req {
     std::map<std::string, std::string> params; // path_params + query_params
     std::map<std::string, std::string> headers; // used by MCP proxy
     std::string path;
     std::string query_string; // query parameters string (e.g. "action=save")
     std::string body;
-    std::map<std::string, raw_buffer> files; // used for file uploads (form data)
+    std::map<std::string, uploaded_file> files; // used for file uploads (form data)
     const std::function<bool()> & should_stop;
 
     std::string get_param(const std::string & key, const std::string & def = "") const {