Consistency improvements for tyro.MISSING

[brentyi]

• More consistent behavior when tyro.MISSING is placed in dataclass definitions
  • Missing semantics were not correctly propagating to children
• More consistent behavior when tyro.MISSING is used inside of struct types
  • For example: field: tuple[DataclassA, DataclassB] = (tyro.MISSING, tyro.MISSING)
  • This was previously throwing an error

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (fd876ed) to head (e98e857).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #264   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          28       28           
  Lines        2250     2251    +1     
=======================================
+ Hits         2247     2248    +1     
  Misses          3        3           

Flag	Coverage Δ
unittests	99.86% <100.00%> (+<0.01%)	⬆️

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