Prevent conflicting profiles in on-demand scans

What does this MR do?

This prevents an on-demand scans from being run with an invalid combination of profiles. When an active scanner profile is selected along with a non-validated site profile, we'll disable the form's submission and show an alert explaining how to fix the conflict.

How to test this?

1. Make sure to enable to the :security_on_demand_scans_site_validation feature flag:

echo "Feature.enable(:security_on_demand_scans_site_validation)" | rails c

2. Make sure you have at least one active scanner profile, you can create one at /:group/:project/-/security/configuration/dast_profiles#scanner-profiles
3. Make sure you have at least one non-validated site profile, you can create one at /:group/:project/-/security/configuration/dast_profiles#site-profiles
4. Visit the On-demand Scans form at /:group/:project/-/on_demand_scans and try selecting an active scanner profile along with a non-validated site profile.

Screenshots (strongly suggested)

conflicting_profiles

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (not required as this is behind a feature flag )
• [-] Documentation (if required) see #271231 (closed)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #280573 (closed)