Update scan result policy editor validation

What does this MR do and why?

Describe in detail what your merge request does and why.

Remove validation of scan result policy yaml

• when moving from yaml mode to rule mode, validation needs to happen on the policy yaml to determine whether rule mode supports the inputted yaml
• when saving a policy, no validation should occur on the frontend because the frontend does not have the official schema. Validation should happen on the backend
• allow for skipping validation of the yaml when converting it to an object on save or to check if rule mode should be disabled
• update initial yaml parsing to use the same logic as when a policy gets updated to ensure parsing is consistent for new and existing policies
• remove the redundant computed property "policyYaml" and update yamlEditorValue directly to have asingle source of truth for what the yaml is
• update tests

What this MR does

• Fixes the drawer and editor not loading for policies with unsupported attributes
• Brings scan result policies code in alignment with scan execution policies code

What this MR does not do

Abstracts similar code between scan execution and scan result policies because

1. This fixes a bug specific to scan result policies
2. This change is complicated enough just with scan result policies
3. This change brings scan result policies code into alignment with scan execution code (Remove validation of scan execution policy yaml... (!98729 - merged)) to make it easier to abstract shared code out in the future
4. This change is already quite large without touching the scan execution code

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 for policies that have supported properties

Scenario	Before	After
Policy Drawer - Yaml with "unsupported" attribute
Policy Editor - Yaml with "unsupported" attribute

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. Turn on the :scan_execution_rule_mode feature flag
3. Navigate to a project => Security & Compliance => Policies => New policy => Scan Execution Policy => yaml mode
4. Copy/paste the below policy into the policy editor and save the policy

Sample Policy

type: scan_result_policy
name: Unsupported policy
description: ''
enabled: true
rules:
  - type: scan_finding
    branches: []
    scanners: []
    vulnerabilities_allowed: 0
    severity_levels:
      - medium
    vulnerability_states:
      - dismissed
actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
      - developer
    unsupported: 'attribute'

5. Navigate to rule mode

• The rule mode should be disabled and the yaml should show in the preview

6. Navigate back to yaml mode => Configure with merge request => Merge the MR => Navigate back to the project => Security & Compliance => Policies => Select the new policy => Edit policy

• The rule mode should be disabled and the yaml should show in the preview

7. Navigate to yaml mode

• the policy should show as expect

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 #396840 (closed)

Related to !98729 (merged)