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

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