Draft: Update scan execution policy validation

What does this MR do and why?

Describe in detail what your merge request does and why.

Update scan execution policy validation

• use schema that is retrieved from the backend and used in the editor to identify if yaml matches schema (e.g. uses allowed values, only includes allows keys) to disable rule mode

Changelog: changed

EE: true

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

No visual change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Navigate to a project => Security & Compliance => Policies => New policy => Scan Execution Policy => yaml mode
3. Type in invalid yaml, navigate to rule mode, verify page is disabled
4. Navigate to a group => Security & Compliance => Policies => New policy => Scan Execution Policy => yaml mode
5. Type in invalid yaml, navigate to rule mode, verify page is disabled

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #369007 (closed)