Fixes saml login flow to work with anonymous auth

[DarshitChanpura]

Description

This PR fixes 2 things:

1. Anonymous login button would not show if only 1 auth option was supplied
2. Broken SAML auth flow when anonymous auth is enabled.
3. Automatic login as anonymous user upon SAML user logout. (Discovered while fixing this bug)

Category

• Bug fix

Why these changes are required?

• For SAML and anonymous auth options to function when both options are enabled
• To prevent auto login as anonymous user upon SAML user logout

What is the old behavior before changes and new behavior after changes?

• At present, SAML auth flow is broken since anonymous auth automatically assumes the Anonymous user when no auth credentials are found. See security plugin PR[1] for more details.

[1] - Companion PR: opensearch-project/security#4152

Issues Resolved

• Resolves [BUG] Having Anonymous and SAML as sign in options does not work. SAML Login fails when Anonymous is enabled #1731
• Resolves [BUG] Automatic login as anonymous user when passing wrong username and password #1840

Testing

• Automated + Manual

Manual testing:

Screen.Recording.2024-04-11.at.11.55.53.AM.mov

Check List

• New functionality includes testing
  - [ ] New functionality has been documented
• Commits are signed per the DCO using --signoff

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