Fix LevelFormatter KeyError on unlisted or custom log levels

[ArmaanjeetSandhu]

LevelFormatter.format() looked up self.formatters[record.levelname] with no fallback, so any record whose level name wasn't an exact key in fmt raised KeyError at log time. This breaks two common cases:

• Custom levels (e.g. the TRACE level shown in docs/custom_level.py).
• Partially-specified fmt dicts, where you only want to override some levels. Every unlisted level then crashes the logging call.

Changes

• LevelFormatter now resolves the formatter with self.formatters.get(record.levelname, self.default_formatter).
• The fallback resolves in this order:
  1. an explicit "DEFAULT" entry in fmt, if provided
  2. otherwise a ColoredFormatter using the default format for the chosen style, inheriting the same **kwargs.
• Updated the LevelFormatter docstring to document the fallback and the "DEFAULT" key.
• Added tests: test_unlisted_level_uses_default (reproduces the original KeyError) and test_explicit_default_key.

I chose a lenient fallback (format the record) over a stricter approach (re-raise with a clearer message), since silently crashing the logging path is the worse outcome and it matches how logging itself tolerates unexpected levels. Happy to switch to a descriptive error instead if strictness is preferred.

[borntyping]

Thanks! Released in 6.12.0.