common/parser: handle reasoning budget (#20297)

* v1

* Finished!

* Handlie cli

* Reasoning sampler

* Apply suggestions from code review

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* Less explosive terminology :)

* Add utf-8 case and tests

* common : migrate reasoning budget sampler to common

* cont : clean up

* cont : expose state and allow passing as initial state

* cont : remove unused imports

* cont : update state machine doc string

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
Co-authored-by: Alde Rojas <hello@alde.dev>

tools/server/server-common.h

@@ -287,6 +287,8 @@ struct server_chat_params {
     bool allow_image;
     bool allow_audio;
     bool enable_thinking = true;
+    int  reasoning_budget = -1;
+    std::string reasoning_budget_message;
     std::string media_path;
 };