special case of datetime as not allowed in date serializer

[davidhewitt]

Change Summary

When serializing date | datetime unions we do not want the date serializer to allow datetime inputs, as they are a lossy serialization.

In general this seems undesirable, so I've gone for the approach of making date serializer disallow datetime always (and emit the usual unexpected type warning).

Related issue number

Related to pydantic/pydantic#7039 (comment)

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[codecov]

Codecov Report

Merging #873 (5fe3498) into main (87b4789) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 5fe3498 differs from pull request most recent head 143d0f8. Consider uploading reports for the commit 143d0f8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #873      +/-   ##
==========================================
- Coverage   93.84%   93.83%   -0.01%     
==========================================
  Files         104      104              
  Lines       15380    15372       -8     
  Branches       25       25              
==========================================
- Hits        14433    14424       -9     
- Misses        941      942       +1     
  Partials        6        6              

Files Changed	Coverage Δ
src/serializers/type_serializers/datetime_etc.rs	98.48% <100.00%> (+0.17%)	⬆️

... and 4 files with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #873 will not alter performance

_{Comparing dh/date-datetime-ser (143d0f8) with main (87b4789)}

Summary

✅ 138 untouched benchmarks

[davidhewitt]

please review