Add dap_powered reviewer assignment options with FF

What does this MR do and why?

Adds the project-level configuration UI and feature flag for the DAP-powered reviewer assignment strategy. No backend wire-up yet — selecting dap_powered is a silent no-op until the follow-up MR wires it up to the execution.

Changes:

• New EE feature flag dap_powered_recommend_reviewers (project actor, WIP).
• New dap_powered entry in REVIEWER_ASSIGNMENT_STRATEGIES.
• Project#dap_powered_recommend_reviewers_available? helper gating on the FF, duo_foundational_flows_enabled, and the recommend_reviewers/v1 flow being enabled for the project.
• Reviewer auto-assignment settings view: switches from a single checkbox to a 3-radio fieldset (Disabled / Code Owners / Duo Agent Platform) when the helper returns true; falls back to the existing checkbox otherwise so default-disabled rollout behaves exactly as today.
• Updates wording around CODEWOWNERS automatic assignment for clarity

NOTE: It needs to be merged after !230863 (merged).

References

• #597374

Screenshots or screen recordings

Before	After
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How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:auto_assign_code_owner_reviewers) # Existing FF this feature is built on top of
   Feature.enable(:dap_powered_recommend_reviewers)

2. Visit Project / Merge Request setting page
3. Verify the new configs are visible and can be changed

MR acceptance checklist

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Related to #597374