Ensure consistent validation

[jonhealy1]

Related Issue(s):

• /collections/{collection_id}/bulk_items doesn't seem to be performing the same validations as /collections/{collection_id}/items #332

Description:

• Bulk insertion via BulkTransactionsClient now strictly validates all STAC Items using the Pydantic model before insertion. Any invalid item will immediately raise a ValidationError, ensuring consistent validation with single-item inserts and preventing invalid STAC Items from being stored. This validation is enforced regardless of the RAISE_ON_BULK_ERROR setting.
• Refactored create_item and update_item methods to share unified logic, ensuring consistent conflict detection, validation, and database operations.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jamesfisher-geo]

Looks great. Just a couple small questions