Fix option_dict_to_string flag serialization (missing f-string) (#804)

For a valueless (flag-style) option, option_dict_to_string emitted the
literal string "{i}" instead of the option name, because the None branch
used a plain string literal rather than an f-string. Flags now serialize
as their name and round-trip cleanly through option_string_to_dict.

Also collapsed the accumulator loop into a single join expression.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DLWoodruff

mpisppy/utils/sputils.py

@@ -944,13 +944,8 @@ def option_dict_to_string(odict):
     """
     if odict is None:
         return None
-    ostr = ""
-    for i, v in odict.items():
-        if v is None:
-            ostr += "{i} "
-        else:
-            ostr += f"{i}={v} "
-    return ostr
+    return "".join(f"{k} " if v is None else f"{k}={v} "
+                   for k, v in odict.items())
 
 
 # Solver-options layered representation. See