Call sync setpoint callbacks with validated value

[GDYendell]

This fix handles a case for the odin meta writer where it returns None for a String datatype. The problem was that it was correctly using the validated value to set the readback, but the original value to set the setpoint.

This also fixes a potential issue that initially thought was the problem where String.length is None and used to truncate strings in the EPICS CA transport and improves some trace messages that made this issue much easier to figure out. Using repr(value) in log messages means strings appear as 'None' instead of None.

It was already doing the right thing in the other case here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.79%. Comparing base (3fe7b0c) to head (9369eba).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files          70       70           
  Lines        2542     2543    +1     
=======================================
+ Hits         2308     2309    +1     
  Misses        234      234           

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