DM-48589: Fix timespans in DimensionRecordTable empty

[dhirving]

Fix an issue where timespans in dimension records read out of DimensionRecordTable would always be empty, due to an error in the deserialization logic.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.47%. Comparing base (3a57374) to head (68342b1).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1150   +/-   ##
=======================================
  Coverage   89.47%   89.47%           
=======================================
  Files         366      366           
  Lines       49115    49123    +8     
  Branches     5955     5955           
=======================================
+ Hits        43944    43952    +8     
  Misses       3753     3753           
  Partials     1418     1418           

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