Make resources per page configurable on admin index pages (#6520)

[PriteshTrivedi]

Summary

Closes #6520.

Makes the number of resources per page on admin index pages globally and locally configurable:

• Adds SolidusAdmin::Config.per_page preference (defaults to 20).
• Updates SolidusAdmin::BaseController#set_page_and_extract_portion_from to pass per_page: self.per_page by default.
• Allows per-controller overrides by overriding def per_page or passing per_page:.
• Removes hardcoded DEFAULT_PER_PAGE from SolidusAdmin::ResourcesController.

Checklist

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.
• I have updated documentation (admin/docs/index_pages.md).
• I have added automated tests to cover my changes.

[Copilot]

Pull request overview

This pull request makes the number of resources shown per page on Solidus Admin index pages configurable via SolidusAdmin::Config.per_page, replacing the previously hard-coded default of 20 in ResourcesController.

Changes:

• Adds a per_page preference to SolidusAdmin::Configuration (default: 20) and tests for it.
• Routes pagination defaults through SolidusAdmin::BaseController#per_page and forwards that into set_page_and_extract_portion_from.
• Documents how to configure the global per-page value and how to override it per controller.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
admin/lib/solidus_admin/configuration.rb	Introduces per_page preference (default 20) for admin index pagination.
admin/app/controllers/solidus_admin/base_controller.rb	Applies SolidusAdmin::Config.per_page as the default per_page: for geared pagination.
admin/app/controllers/solidus_admin/resources_controller.rb	Removes controller-local default per-page constant/method (now uses the base/controller config behavior).
admin/spec/solidus_admin/configuration_spec.rb	Adds specs asserting default and configurable per_page.
admin/spec/controllers/solidus_admin/base_controller_spec.rb	Adds specs asserting per_page comes from config and is passed into geared pagination.
admin/docs/index_pages.md	Documents the new global setting and per-controller overrides.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.16%. Comparing base (1f5bf5c) to head (c7adb3e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6543      +/-   ##
==========================================
+ Coverage   91.82%   92.16%   +0.33%     
==========================================
  Files         879     1037     +158     
  Lines       17772    21197    +3425     
==========================================
+ Hits        16320    19536    +3216     
- Misses       1452     1661     +209     

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