[Bug] Fix path to built-in Vizro assets

[antonymilne]

Description

There was a subtle bug where we were using routes_pathname_prefix somewhere we should have used requests_pathname_prefix. What this meant was that in some deployments our built-in Vizro js and css would not be found.

• requests_pathname_prefix is "is a local URL prefix for file requests. Must end with routes_pathname_prefix" (from Dash docs). "the prefix for the AJAX calls that originate from the client (the web browser)" (from comment in Dash source code). This is what's used when injecting flask server into Dash as in Host on a route of existing Flask app
• routes_pathname_prefix is "a local URL prefix for JSON requests. Must start and end with '/'" (from Dash docs). "the prefix for the API routes on the backend (this flask server)" (from comment in Dash source code). This is what's used when mounting Dash onto a server using middleware (could be WSGI like flask or ASGI like FastAPI that uses WSGIMiddleware) as in DispatcherMiddleware
• setting url_base_pathname = x is equivalent to setting both requests_pathname_prefix = x and routes_pathname_prefix = x

Previously things worked ok when requests_pathname_prefix == routes_pathname_prefix but not otherwise. Now it works ok - tested on:

• injecting flask server into Vizro
• mounting Vizro into Flask app
• mounting Vizro into FastAPI app

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[huong-li-nguyen]

LGTM 👍