fix: attributes

[akrivi]

This PR:

1. Updates copy_object to also copy object-level attributes (t_attribute_data) in addition to memberships and properties.
   This is particularly important for classes such as ClassEnum.Horizon, where key defining characteristics (e.g. Date From, Step Count, Chrono Date From, Look-ahead At a Time) are stored as attributes. Previously, these were not copied, resulting in incomplete duplicated objects.

2. Introduces add_attributes_from_records for efficient bulk insertion of attribute values.

3. Implements delete_attribute, get_attributes, check_attribute_exists

4. Documentation updates related to attributes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.97959% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.93%. Comparing base (3003e52) to head (bb913f3).

Files with missing lines	Patch %	Lines
src/plexosdb/utils.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   95.55%   95.93%   +0.37%     
==========================================
  Files           8        8              
  Lines        1823     1919      +96     
==========================================
+ Hits         1742     1841      +99     
+ Misses         81       78       -3     

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[pesap]

Left 3 inline comments. Main fixes requested: early validation for explicit attribute records missing name, plus regression test for that case. I also linked follow up issue #132 for chunking standardization so we can keep this PR focused.

[pesap]

@akrivi , is this ready for review?

[akrivi]

yes!

[pesap]

Approved after fixing the broken MyST/Sphinx API reference link in docs/source/howtos/add_attributes.md and validating the docs build locally.