Update DCR prompt to include full files

What does this MR do and why?

• This MR updates the DCR prompt to send full files attached at the bottom of the prompt
• Prompt changes are behind the feature flag, duo_code_review_full_file
• Feature flag rollout issue: [Feature flag] Rollout of `duo_code_review_full... (#537556 - closed)

How to set up and validate locally

Prerequisites:

• Make sure the AI features are enabled locally using this guide
• Enable duo_code_review_full_file feature flag for the user
• Open a MR with some files where context from full files in one-go is relevant such as adding a duplicate method with the same name.
• Assign GitLabDuo as a reviewer either by commenting /assign_reviewer @GitLabDuo or select GitLabDuo from the reviewers dropdown.

Validate:

• Make sure GitLabDuo works as expected without duo_code_review_full_file being enabled.
• Enable duo_code_review_full_file and make sure GitLabDuo calls out adding a method with a duplicated name (btw this context wouldn't be present in the diffs so it's a great test case to test).
• Make sure there are no unwanted comments from GitLabDuo.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #510266 (closed)