Add duo_foundational_flows_enabled checks for duo code review DAP flow

What does this MR do and why?

This MR adds duo_foundational_flows_enabled as a required condition for Duo Code Review to be available via the Duo Agent Platform (DAP) flow.

Background: Duo Code Review can operate through two flows:

• Classic flow (Duo Enterprise): Always available when the add-on is active, regardless of feature flags
• DAP flow (Duo Pro/Core): Requires multiple conditions to be met

This change adds duo_foundational_flows_enabled as a prerequisite for the DAP flow, ensuring that the foundational flows infrastructure is properly enabled before allowing Duo Pro/Core customers to use Code Review features.

How to set up and validate locally

1. Verify Duo Enterprise still works (unaffected by new setting):

   ApplicationSetting.current.update!(duo_features_enabled: true, duo_foundational_flows_enabled: false)
   # Add Duo Enterprise add-on: GITLAB_SIMULATE_SAAS=1 bundle exec 'rake gitlab:duo:setup' 
   ApplicationSetting.current.auto_duo_code_review_settings_available? # => true

2. Verify Duo Core blocked when duo_foundational_flows_enabled = false:

   ApplicationSetting.current.update!(duo_foundational_flows_enabled: false, instance_level_ai_beta_features_enabled: true)
   Feature.enable(:duo_code_review_on_agent_platform)
   # Add Duo Core add-on: GITLAB_SIMULATE_SAAS=1 bundle exec 'rake gitlab:duo:setup[core]'
   ApplicationSetting.current.auto_duo_code_review_settings_available? # => false

3. Verify Duo Core works when duo_foundational_flows_enabled = true:

   ApplicationSetting.current.update!(duo_foundational_flows_enabled: true)
   ApplicationSetting.current.auto_duo_code_review_settings_available? # => true

4. Verify project-level override works:

   # Namespace enabled, project disabled
   group.namespace_settings.update!(duo_foundational_flows_enabled: true)
   project.project_setting.update!(duo_foundational_flows_enabled: false)
   project.auto_duo_code_review_settings_available? # => false

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Related to Integrate Duo Code Review into Duo Agent Platform (&18645)