Enable `assumeImmutableResults` in dast_site_validation provider

What does this MR do?

#331283 (closed)

Enable assumeImmutableResults to enforce immutability

Page loads fine without console errors.

Screenshots or Screencasts (strongly suggested)

before	after
dast-validation-before	dast-validation-after

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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