feat(msgspec_dto): include msgspec tagged union field

[abdulhaq-e]

When using msgspec tagged union, it is sometimes desirable to include the tag field in the DTO. This wasn't supported.

The current implementation unconditionally includes this field. I believe it's a good default option, however I think we should introduce a custom MsgspecDTOConfig (similar to what we had for SQLAlchemy) which can control this behaviour.

This code was also generated using AI but this sometimes it's an area of the code base I'm more familiar with.

I've created it as draft to discuss having MsgspecDTOConfig and the flag and for any further edits.

[abdulhaq-e]

This is similar to #3742 which added the tag field to open api. Perhaps since it's always added to Open api and since this is the new v3, it might be okay to just introduce it unconditionally. One can then exclude it if needed using exclude in DTOConfig

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.35%. Comparing base (69a1ad9) to head (2a14993).

Files with missing lines	Patch %	Lines
litestar/dto/msgspec_dto.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4671      +/-   ##
==========================================
+ Coverage   67.33%   67.35%   +0.01%     
==========================================
  Files         292      292              
  Lines       14941    14957      +16     
  Branches     1676     1677       +1     
==========================================
+ Hits        10061    10074      +13     
- Misses       4743     4746       +3     
  Partials      137      137              

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