server: frame the router child state command as a whole line (#28747)

The child writes its state commands on stdout while the logger writes
on stderr, and both share a single pipe. The logger emits the trailing
color reset after the newline of a debug, warn or error entry, so that
escape sequence has no newline of its own and the router reads it glued
in front of the next command. The line prefix check then fails and the
command is forwarded as a log line instead of being handled, which
leaves a finished download stuck in the downloading state.

Writing the command with a leading newline closes the pending line so
it always starts at a line boundary.

tools/server/server-models.cpp

@@ -1786,7 +1786,10 @@ void server_child::notify_to_router(const std::string & state, const json & payl
     std::lock_guard<std::mutex> lk(mtx_stdout);
     common_log_pause(common_log_main());
     fflush(stdout);
-    fprintf(stdout, "%s%s\n", CMD_CHILD_TO_ROUTER_STATE, safe_json_to_str(data).c_str());
+    // the router matches the command on a line prefix, so the leading newline
+    // closes whatever the logger left open on the shared pipe, down to the
+    // trailing color reset that carries no newline of its own
+    fprintf(stdout, "\n%s%s\n", CMD_CHILD_TO_ROUTER_STATE, safe_json_to_str(data).c_str());
     fflush(stdout);
     common_log_resume(common_log_main());
 }