make id gap a UndulatorGapMotor

[Relm-Arrowny]

Refactored UndulatorGap to inherit directly from UndulatorGapMotor, removing the nested self.motor wrapper and simplifying signal delegation.

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (145b592) to head (3163c4f).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           update_id_logic_to_use_standard_movable    #2167   +/-   ##
========================================================================
  Coverage                                    99.15%   99.16%           
========================================================================
  Files                                          356      356           
  Lines                                        13871    13863    -8     
========================================================================
- Hits                                         13754    13747    -7     
+ Misses                                         117      116    -1     

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