Chore: Write E2E tests for "Allow admin to set Team-specific billing overrides"

### Description

The feature "Allow admin to set Team-specific billing overrides" exists but lacks end-to-end test coverage. This would be the first step into refactoring the admin form

E2E tests should cover the key flows of setting and modifying team-specific billing overrides to ensure reliability and prevent regressions.

### Epic/Story

https://github.com/FlowFuse/flowfuse/issues/5794

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate