fix(external-api): prevent unload listener accumulation on SET_CONFIG

[codewithabhay10]

Motivation and Context

Fixes #17245

Currently, the SET_CONFIG middleware in the External API registers an anonymous unload (or beforeunload) event listener every time it is dispatched:
https://github.com/jitsi/jitsi-meet/blob/master/react/features/external-api/middleware.ts#L244

Because the handler is an anonymous function, the listeners accumulate over the lifecycle of a long-lived session if SET_CONFIG is updated multiple times, leading to redundant notifyConferenceLeft and dispose() calls on window unload.

Description

This PR resolves the issue by hoisting the unload handler into module-level variables (_location and _unloadHandler).
Before registering a new listener during the SET_CONFIG dispatch, any previously registered listener is removed using window.removeEventListener(). This ensures that exactly one unload listener exists at any given time.

Testing Performed

• Validated that dispatching SET_CONFIG no longer creates orphaned unload event listeners on the window object.
• Validated that when the page unloads, notifyConferenceLeft() and dispose() are gracefully executed exactly once.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.