chore(core): Test saml with the UI based config. Display modal when config has changed without saving

[everettbu]

Mirror of n8n-io/n8n#26141
Original author: guillaumejacquart

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Summary

• Guard getIdentityProviderInstance() against empty metadata to prevent cryptic ERR_IDP_METADATA_MISSING_SINGLE_SIGN_ON_SERVICE errors from samlify
• Change SAML test endpoint from GET to POST, accepting the current form values (metadata/metadataUrl) so users can test their IdP connection without saving first
• Add unsaved changes prompt on the SSO settings page when navigating away with modified config (Save and leave / Leave without saving / Keep editing)
• Refactor getLoginRequestUrl and fetchMetadataFromUrl to support both production and test flows without code duplication

Screen of the modal inviting to save on change

https://www.loom.com/share/41ba37232d9a403782714a1f88a95139

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/IAM-259/saml-sso-google-workspace-idp-metadata-xml-fails-to-parse-with-err-idp

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)