[fix](fe) Remove oidc plugin assertions from AuthenticationPluginManagerTest

[heguanhui]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: AuthenticationPluginManagerTest asserts the existence of an OIDC authentication plugin (oidc), but no OIDC plugin implementation exists in the codebase. The OidcPluginFactory class referenced in AuthenticationPluginFactory.java comments is only a documentation example with no actual implementation, module, or SPI registration. This causes testPluginsAutoLoaded and testGetFactory to fail.

Release note

None

Check List (For Author)

• Test
  • Unit Test
• Behavior changed:
  • No.
• Does this need documentation?
  • No.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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