Fix false positives in Rails/Pluck for non-AR/Array receivers

[dfl]

Summary

Adds an AllowedReceivers config option to Rails/Pluck that skips the cop when any method in the receiver's call chain matches the list. The default list includes the most common Nokogiri traversal methods (xpath, css, search, at_xpath, at_css).

Motivation

Rails/Pluck rewrites .map { |el| el[:key] } to .pluck(:key) without inspecting the receiver. This produces broken autocorrections when the receiver isn't an ActiveRecord::Relation or Array — for example, Nokogiri:

# before autocorrect (works)
doc.xpath('//item').map { |node| node['name'] }

# after autocorrect (NoMethodError: undefined method `pluck` for Nokogiri::XML::NodeSet)
doc.xpath('//item').pluck('name')

The cop is documented as @safety: unsafe, but Nokogiri is widespread enough that a default-on guard seems worth it. Other rubocop-rails cops (Rails/RedundantActiveRecordAllMethod, Rails/DynamicFindBy, Rails/SaveBang) already use an AllowedReceivers config for similar reasons.

Behavior

• AllowedReceivers is matched against any send/csend node anywhere in the receiver chain, by method name. This handles both immediate (page.css('.row').map { ... }) and chained (doc.xpath('//x').first_thing.map { ... }) receivers.
• Users can extend the list for other libraries that expose [] on non-Array collections (HappyMapper, Mongoid documents, etc.) without further code changes.

Changes

• lib/rubocop/cop/rails/pluck.rb: add allowed_receiver? guard and early-return in on_block.
• config/default.yml: add AllowedReceivers default for Rails/Pluck.
• spec/rubocop/cop/rails/pluck_spec.rb: 4 new examples covering immediate, chained, safe-nav, and a negative (AR chain still flagged).
• changelog/...: changelog entry.

Test plan

• bundle exec rspec spec/rubocop/cop/rails/pluck_spec.rb — 40/40 passing
• bundle exec rspec — full suite green (modulo the changelog PR-number format check, which will pass once the PR number is filled in)
• Reviewer confirms AllowedReceivers default list matches what the team is comfortable shipping by default