Rename code_owner_reviewer_assignment_strategy to reviewer_assignment_strategy

Summary

Renames the code_owner_reviewer_assignment_strategy column and all associated model references to the more generic reviewer_assignment_strategy. This prepares the reviewer assignment infrastructure to be reused by DAP (Duo Agent Platform) and other approval types beyond CODEOWNERS.

What does this MR do?

• Migration: Renames project_settings.code_owner_reviewer_assignment_strategy to reviewer_assignment_strategy
• Model: Renames constant CODE_OWNER_REVIEWER_ASSIGNMENT_STRATEGIES to REVIEWER_ASSIGNMENT_STRATEGIES, updates the enum and attribute declarations
• Method: Renames code_owner_reviewer_auto_assignment_enabled? to reviewer_auto_assignment_enabled?
• Specs/config: Updates all references in specs and API attributes YAML

Why?

The DAP epic (&20711 - "DAP-Powered Intelligent Reviewer Assignment") depends on the assignment infrastructure from Epic &20708 ("Native CODEOWNERS Reviewer Auto-Assignment"). DAP will add its own strategy (e.g., duo_intelligent) to select optimal individual reviewers from approval groups. By making the column/enum generic now, we avoid a breaking rename later.

Related

• Epic: &20708
• DAP Epic: &20711
• Part 1 (DB migration): !224175 (merged)
• Part 2 (Service): !224176 (merged)