fix: copy_object date range metadata copying

[Hasakev]

Reopens #139
Resolves #138

Summary

This pull request reapplies the changes from #139 to address #138. It improves the object-copying logic so that date-bound property values are preserved when objects are duplicated.

Previously, date_from and date_to values associated with object properties were not copied to the newly created object. This could result in copied objects losing important temporal property metadata.

Changes

Object property copying

• Updated _copy_object_properties in src/plexosdb/db.py to explicitly copy date_from and date_to values when duplicating object properties.
• Ensures that copied objects retain the same effective date ranges as the source object.

Tests

• Added test_copy_object_copies_date_from_and_date_to in tests/test_plexosdb_copy_object.py.
• Verifies that date_from and date_to are preserved when copying an object.
• Added the required datetime import to support the new test case.

Notes

This PR replaces the previous attempt in #139 after the changes were reapplied cleanly.

[pesap]

Thanks @Hasakev !