refactor: Use pydantic event model for start and stop document handling

[abbiemery]

This is a change on top of this #1221 pr to see if using the pydantic models was better. I don't know about their support in the larger bluesky ecosystem to make any comment on their use here, but was worth playing with.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.69%. Comparing base (43bc037) to head (3ad51b9).

Additional details and impacted files

@@               Coverage Diff                @@
##           clear-start-doc    #1247   +/-   ##
================================================
  Coverage            94.69%   94.69%           
================================================
  Files                   41       41           
  Lines                 2619     2620    +1     
================================================
+ Hits                  2480     2481    +1     
  Misses                 139      139           

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[DiamondJoseph]

Since we're almost immediately model_dumping the BaseModel and working with the arbitrary extra fields, our deserialisation doesn't appear to play nicely and it changes the behaviour of a null scan_id I think let's merge #1221 as is and leave this for now.