Change ArtifactResponse extracting method

Earlier a regex pattern was used to extract the ArtifactResponse object. Hardcoded regex string can cause problems when specification versions or lib versions are upgraded. Therefore we have used library constants to validate and extract the object.

Refers wso2/product-is#3401

ASAP

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[ ] Are there sufficient test cases? Ensure that all components are tested individually; models, forms, and serializers should be tested in isolation even if a test for a view covers these components.
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wso2-extensions / identity-outbound-auth-samlsso