[8.x] Restore 204 response for login and logout endpoints

[JoshSalway]

Summary

• Restores response(status: 204) return from UserController::login() and UserController::logout() that was present in 7.x (PR Fix favicon 404s triggered when using loginAs() or logout() methods #1061) but lost during the 8.x branch creation
• Without the 204 response, WebDriver waits indefinitely for page readiness after auth actions, causing intermittent loginAs() failures (#1152) and unnecessary favicon 404 requests

Problem

The fix from PR #1061 (commit 076865d) correctly returned a 204 No Content response from both endpoints to signal to WebDriver that navigation is complete with no content to render. This fix exists on 7.x and master but was never ported to 8.x — the file was recreated from the pre-fix version.

Without the explicit 204 response:

1. Laravel returns an implicit 200 with empty body
2. WebDriver waits for document readyState, favicon loads, etc.
3. Subsequent browser actions may execute before auth session is fully established
4. Results in intermittent loginAs() failures in CI

Test plan

• All 199 existing tests pass locally
• Diff matches the original Fix favicon 404s triggered when using loginAs() or logout() methods #1061 fix exactly
• Verified fix only exists on 7.x/master branches, not 8.x

🤖 Generated with Claude Code

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