WA-DOC-027: Add any_of Mongoid 8 migration pattern doc

[kitcommerce]

Summary

WA-VERIFY-091 (PR #1091) found and fixed two real bugs related to any_of semantics changing between Mongoid 7 and Mongoid 8. Document this as a reusable migration pattern for downstream Workarea implementations.

Objective

Create docs/rails7-migration-patterns/mongoid8-any-of-semantics.md to describe the any_of scoping change, detection strategy, and fix pattern — so Workarea plugin authors and downstream apps can apply the same fixes.

Approach

Base the doc on the notes from PR #1091 (notes/WA-VERIFY-091-mongoid8-any-of.md if it exists) and the fix patterns implemented in:

• admin/app/view_models/workarea/admin/activity_view_model.rb
• core/app/models/workarea/tax/rate.rb

Acceptance Criteria

• New file: docs/rails7-migration-patterns/mongoid8-any-of-semantics.md
• Describes the Mongoid 7 vs 8 any_of API difference (array arg vs splat)
• Includes detection grep command (rg ".any_of" --glob='*.rb')
• Documents the fix patterns (splatting, loop-to-collect refactor)
• Links to PR WA-VERIFY-091: Audit and fix Mongoid 8 any_of scoping semantics #1091 as the reference implementation
• README index in docs/rails7-migration-patterns/README.md updated to include new doc

Verification Plan

• Read the doc for clarity and completeness
• Verify README index updated

Client Impact

None (documentation only).