MONGOID-5684 Ensure field_was returns the same type as field for hash-typed fields

[jamis]

In Mongoid, Hash-typed fields return BSON::Document instances (a special Hash subclass). However, assigning a new hash value to such a field and then querying its previous value (via [field]_was) would return a Hash instance, even though it was actually a BSON::Document instance previously.

This PR fixes that by implementing an appropriate deep copy method for BSON::Document instances.

[Copilot]

Pull request overview

Fixes Mongoid dirty-tracking so that [field]_was preserves BSON::Document for Hash-typed fields (matching the field getter), by adding a BSON::Document#__deep_copy__ implementation and a regression spec.

Changes:

• Add BSON::Document#__deep_copy__ that deep-copies while preserving the BSON::Document type.
• Load the new BSON::Document extension via lib/mongoid/extensions.rb.
• Add a regression spec asserting map_was returns a BSON::Document and the expected prior value.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
spec/mongoid/changeable_spec.rb	Adds regression coverage for *_was behavior on Hash fields stored as BSON::Document.
lib/mongoid/extensions/bson_document.rb	Introduces BSON::Document#__deep_copy__ to preserve type during deep copy for dirty tracking.
lib/mongoid/extensions.rb	Requires the new BSON::Document extension so it is loaded with other core extensions.

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