Handle ValueError for missing format keys

[cpruijsen]

Catch (KeyError, ValueError) in _format_by_dict so FluentRecordFormatter(fill_missing_fmt_key=True) fills missing { and $ style fields with None on Python 3.8+. Since Python 3.8, logging.StrFormatStyle / StringTemplateStyle wrap a missing field as ValueError instead of KeyError (python/cpython#79025). _format_by_dict only caught KeyError, so the fill flag did nothing for those styles. % style was already handled because it still uses value % record.__dict__.

Fixes #189

Decision

Catch (KeyError, ValueError) in the existing handler, as asked in the issue ("ValueError must be handled here"). The other option is to treat ValueError as a missing key only when __context__ is KeyError, so an invalid format spec (e.g. {name:d} on a str) still raises when fill_missing_fmt_key=True. _format_by_dict already uses a bare type catch; the flag is opt-in; inspecting __context__ is more clever than the rest of handler.py. Can switch to the narrower match.

Test plan

• pytest tests/test_handler.py (20 passed, including new {/$ fill tests)
• New tests fail on unfixed _format_by_dict (ValueError: Formatting field not found in record: 'custom_field'), each in isolation
• pytest tests/test_asynchandler.py (15 passed; formatter is shared)
• CI: Python 3.7-3.12 / PyPy (3.7 still raises KeyError for {/$; both types are caught)