fix: write annotations for all value types in serde serializer

[popematt]

Issue #, if available:

N/A

Description of changes:

In the course of upgrading to rc12, a regression was discovered for the serde feature. Some value types include annotations, and some don't, and it was affecting the enum variant serialization. This ensures that annotations are always included, and adds a bunch more tests to make sure this doesn't happen in future.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.14%. Comparing base (96d4ff5) to head (20162a8).

Files with missing lines	Patch %	Lines
src/serde/ser.rs	57.14%	0 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
+ Coverage   78.97%   79.14%   +0.16%     
==========================================
  Files         140      140              
  Lines       34822    34911      +89     
  Branches    34822    34911      +89     
==========================================
+ Hits        27502    27630     +128     
+ Misses       5245     5194      -51     
- Partials     2075     2087      +12     

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