`datetime_format=` does not apply to custom formatters

When a custom formatter that responds to datetime_format= is set on a Logger, calling Logger#datetime_format= applies the format to the internal @default_formatter instead of the active formatter. This means the datetime format has no effect on log output.

Steps to reproduce

require 'logger'

custom_formatter = Logger::Formatter.new
logger = Logger.new($stdout, formatter: custom_formatter, datetime_format: "%d%b%Y@%H:%M:%S")

# Expected: datetime_format is set on custom_formatter
logger.datetime_format  # => nil (unexpected)
custom_formatter.datetime_format  # => nil (unexpected)
logger.info("hello")
# Outputs default datetime format instead of "%d%b%Y@%H:%M:%S"

The same issue occurs when setting datetime_format after initialization:

logger = Logger.new($stdout, formatter: Logger::Formatter.new)
logger.datetime_format = "%d%b%Y@%H:%M:%S"
logger.info("hello")
# Still uses default datetime format

Root cause

Logger#datetime_format= always delegates to @default_formatter:

def datetime_format=(datetime_format)
  @default_formatter.datetime_format = datetime_format
end

But format_message uses @formatter when set:

def format_message(severity, datetime, progname, msg)
  (@formatter || @default_formatter).call(severity, datetime, progname, msg)
end

Additionally, in initialize, datetime_format is set before the custom formatter is assigned, so it never reaches the custom formatter even during construction.

Expected behavior

datetime_format= should apply to the active formatter (custom or default) if it responds to datetime_format=.

Start by reading Logger#datetime_format=, initialize, and format_message, using the reproduction steps to trace which formatter receives the setting. Add regression coverage for custom formatters during and after initialization, and verify that log output uses the requested datetime format.