MONGOID-5900 Fix #pluck on associations

[jamis]

This addresses a few bugs with the implementation of #pluck on associations.

1. It now respects localized fields, and the current localization settings.
2. It now plucks loaded and unloaded records from the association.
3. It now does the right thing when fields are referenced by an alias.

Tests from #6044 (from @johnnyshields) are included in this PR (thank you, Johnny!).

[Copilot]

Pull Request Overview

This PR fixes several bugs in the #pluck implementation for associations in Mongoid. It adds proper support for localized fields (respecting current locale and fallbacks), handles both loaded and unloaded association records correctly, and properly resolves field aliases.

Key changes:

• Extracts common pluck functionality into a new Pluckable module for code reuse
• Updates has_many and embeds_many associations to properly handle plucking from mixed loaded/unloaded/added documents
• Adds comprehensive test coverage for localized fields, aliased fields, and embedded documents

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
lib/mongoid/pluckable.rb	New shared module providing pluck functionality with support for field aliases, localization, and demongoization
lib/mongoid/contextual/mongo.rb	Refactored to use the new Pluckable module, removing duplicated code
lib/mongoid/association/referenced/has_many/enumerable.rb	Added pluck implementation that handles loaded, unloaded, and added documents
lib/mongoid/association/referenced/has_many/proxy.rb	Added pluck delegation to the enumerable target
lib/mongoid/association/embedded/embeds_many/proxy.rb	Added pluck delegation to criteria alongside existing find delegation
spec/support/models/*.rb	Added test models with localized and aliased fields to support comprehensive testing
spec/mongoid/association/referenced/has_many/enumerable_spec.rb	Added extensive test coverage for pluck functionality including localization, aliases, and embedded documents

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[johnnyshields]

Hi Jamis, great, thank you for handling this.

I noticed you extracted out a Pluckable concern. After merging this PR, I would encourage you to go a step further and consider merging my PluckEnumerator and #pluck_each PR here: #5497

PluckEnumerator is a big improvement, not only in terms of code organization, but also behavior. Currently, #pluck force-loads the plucked values for all documents in the query. So if you are pluck-ing 1,000,000 docs, you need to load all 1,000,000 values into Ruby's memory, which will cause an OOM crash of your app. #pluck_each behaves like Mongoid's #each iterator--it loads the plucks in batches and GC's them. I use it in dozens of places in my app's code.