Customer chat: remaining any in the attachment click handler and new tests

[marassteiner]

Follow-up from #1297 (not a merge blocker). Style / type-safety only.

Evidence

src/screens/chat.screen.tsx:761 still uses React.MouseEvent<any>. That signature was already on the merge base.

The new src/__tests__/chat.screen.test.tsx adds further any (for example the support-issue fixture around line 19 and untyped component mocks around line 107).

Suggested fix

Type the click handler as React.MouseEvent<HTMLElement> (or React.SyntheticEvent). Give the test fixtures a small local interface instead of any.