Keycloak Auth endpoint sends a 405 Method Not Allowed on Preflight Request

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### Area

adapter/javascript

### Describe the bug

Keycloak returns a 405 Method Not Allowed on an OPTION preflight request after an express server tries to forward an unauthenticated user from the browser to the keycloak login page.

### Version

26.0.5

### Regression

- [ ] The issue is a regression

### Expected behavior

My React frontend sends a request to a secured endpoint in my express proxy. The proxy returns a 302 Found to the frontend. The frontend sends a preflight request that returns the Access-Control-Allow-Origin header and forwards my frontend to the login page.

### Actual behavior

When the preflight request happens the request returns a 405 Method Not Allowed error and lables it as a CORS Missing Allow Origin, since the Access-Control-Allow-Origin header is not returned.

### How to Reproduce?

Send a Get request from a react frontend to a keycloak-connect protected express endpoint.

### Anything else?

_No response_

Start in the adapter/javascript area and reproduce the OPTIONS preflight from a React frontend through the protected Express endpoint. Compare the 405 response with the expected redirect and CORS headers; done means the preflight no longer fails as described.

Written by the indexing model from the issue text.