Fix msgspec template to add field

[ianbuss]

With msgspec outputs, the order of generated fields sometimes results in invalid class definitions, with required and optional fields in the wrong order. Additionally, if using --snake-case-field, field aliases are not added.

This PR contains the following fixes:

• The _has_field_assignment sort function is modified to sort optional fields last
• The __str__ function for msgspec fields is updated to add an explicit None default for optional fields when no other default has been set.
• The jinja template is modified to write out the field.field for annotated types.
• Added a new unit test to and modified existing tests in test_main.py.

Fixes #1943
Fixes #1919
Fixes #1920

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1942   +/-   ##
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  Coverage        ?   98.90%           
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  Files           ?       37           
  Lines           ?     4186           
  Branches        ?      973           
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  Hits            ?     4140           
  Misses          ?       29           
  Partials        ?       17           

Flag	Coverage Δ
unittests	98.56% <100.00%> (?)

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[codspeed-hq]

CodSpeed Performance Report

Merging #1942 will not alter performance

_{Comparing ianbuss:retain-ordering-of-fields-with-snake-casing (72edb93) with main (01dd102)}

Summary

✅ 29 untouched benchmarks