Fix option_dict_to_string flag serialization (missing f-string)

[DLWoodruff]

What

sputils.option_dict_to_string serialized a valueless (flag-style) option — one whose value is None — as the literal string {i} instead of the option's name. The None branch used a plain string literal where an f-string was intended:

if v is None:
    ostr += "{i} "        # emitted literal "{i}"
else:
    ostr += f"{i}={v} "

So, e.g., option_dict_to_string({'LogFile': None, 'mipgap': 0.01}) returned '{i} mipgap=0.01 '.

Fix

Use an f-string for the flag branch so flags serialize as their name and round-trip cleanly back through option_string_to_dict. The accumulator loop is collapsed into a single join while here.

return "".join(f"{k} " if v is None else f"{k}={v} "
               for k, v in odict.items())

Behavior is unchanged for the other cases and preserves the docstring's "None begets None, empty begets empty" contract:

input	output
None	None
{}	''
{'LogFile': None}	'LogFile '
{'mipgap': 0.01}	'mipgap=0.01 '
{'LogFile': None, 'mipgap': 0.01, 'threads': 4}	'LogFile mipgap=0.01 threads=4 '

All round-trip back through option_string_to_dict.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.18%. Comparing base (3174edf) to head (20f844a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #804      +/-   ##
==========================================
- Coverage   76.19%   76.18%   -0.01%     
==========================================
  Files         169      169              
  Lines       22268    22263       -5     
==========================================
- Hits        16966    16960       -6     
- Misses       5302     5303       +1     

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