[Bug]: SAML POST Binding not handled correctly from StartIdentityProviderIntent

[stebenz]

Preflight Checklist

• I could not find a solution in the documentation, the existing issues or discussions
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Version

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Describe the problem caused by this bug

The endpoint StartIdentityProviderIntent returns either a nextStep as authUrl or postForm. Currently the authUrl is used correctly to redirect too, but the postForm is never handled. As a result the external SAML IDPs can only be added with Redirect-Binding for now.

To reproduce

Steps to reproduce:

1. add external SAML IDP with Post-Binding
2. try to login from typescript login through external SAML IDP
3. no redirect

Screenshots

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Expected behavior

To solve this the postForm has to be displayed directly form the response, which has an autosubmit as redirect to the IDP.

Relevant Configuration

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Additional Context

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