common/parser: add --skip-chat-parsing to force a pure content parser. (#20289)

* Add `--force-pure-content` to force a pure content parser.

* Update common/arg.cpp

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

* Change parameter name [no ci]

---------

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

tools/server/server-context.cpp

@@ -911,6 +911,7 @@ private:
                 /* reasoning_budget      */ params_base.reasoning_budget,
                 /* reasoning_budget_msg  */ params_base.reasoning_budget_message,
                 /* media_path            */ params_base.media_path,
+                /* force_pure_content    */ params_base.force_pure_content_parser
             };
         }