add per_step timeout to i06 vector magnet

[Relm-Arrowny]

Fixes #2166

Changes:

• Added calculate_timeout_per_step in SuperConductingMagnetController
• Modified SuperConductingMagnetControllerto make use of calculate_timeout_per_step
• Added get_ramp_rate to ThreeMagnetAxisPowerSupply

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 99.18%. Comparing base (b4b38f1) to head (92baa83).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2170   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files         360      360           
  Lines       14296    14306   +10     
=======================================
+ Hits        14180    14190   +10     
  Misses        116      116           

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