Add validation to ensure calendar event time is greater or equal to device time for protocol 7

[synthead]

Related to #316.

The DSI e-BRAIN (protocol 7) uses 5 minute intervals relative to the current date of the device time as the event time. This means that if the device time is midnight, and the event if at 00:15, the calendar event time is sent as "3". If the event is midnight on the next day, it is sent as "288" (1440 minutes in a day / 5 minutes).

Since this is relative to the device time, if a calendar event happens to be before the device time, then this method will produce negative values in its array of bytes:

timex_datalink_client/lib/timex_datalink_client/protocol_7/eeprom/calendar/event.rb

Lines 26 to 32 in f80d9d1

	def time_formatted(device_time)
	device_time_midnight = Time.new(device_time.year, device_time.month, device_time.day)
	seconds = (time - device_time_midnight).to_i
	five_minutes = seconds / FIVE_MINUTES_SECONDS
	lsb_msb_format_for(five_minutes)
	end

When this happens, this exception is seen when attempting a transfer:

/home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:35:in `chr': -261 out of char range (RangeError)
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:35:in `block (2 levels) in write'
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:32:in `each'
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:32:in `block in write'
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:31:in `each'
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client/notebook_adapter.rb:31:in `write'
        from /home/jac/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/timex_datalink_client-0.12.2/lib/timex_datalink_client.rb:113:in `write'
        from ebrain.ruby:111:in `<main>'

Since it is not possible to transfer calendar events in the past on protocol 7 due to the relative time comparison, an ActiveModel validation should be added to protocol 7's Eeprom::Calendar::Event. This will ensure that invalid data is not passed to protocol 7's Eeprom, and ultimately causing exceptions when attempting to pass negative values to TimexDatalinkClient::NotebookAdapter.