fix(routing): normalize trailing slashes on mounted routes

[pctablet505]

Fixes #869

Mount('/users', routes=[Route('/', users), Route('/{username}', user)]) currently only matches when the request path ends with a slash. GET /users returns a 307 redirect to /users/, and GET /users/bob/ returns a 307 redirect to /users/bob, while a top-level Route('/', ...) responds to both / and an empty path. This asymmetry between Route and Mount has been reported repeatedly: external webhook callers that don't follow redirects fail, Ariadne's GraphQL mount is unreachable at /graphql, and people have resorted to setting app.router.redirect_slashes = False or adding middleware to rewrite the path.

In #823 the maintainer explained that requiring the slash was intentional, because a mount point is meant to be /path-to-mount/{...} and /some-path should not match /some-path{...}. That constraint is still respected here: /usersa still 404s.

This change rewrites the catch-all path regex that Mount builds from compile_path(self.path + "/{path:path}") so the trailing slash is optional. When the mount point is hit without a slash, the router appends the slash to scope["path"] internally before dispatching to the mounted app, so Route('/', ...) resolves directly and Route('/{username}', ...) works with or without a trailing slash. The existing test_mount_urls assertion is updated to expect /users to stay /users instead of redirecting to /users/, and a new test_mount_trailing_slash_with_child_routes covers the child-route cases.

The main trade-off is that /users no longer redirects to /users/; it now serves the mounted content directly. That removes the 307 pain for mounts but changes behavior for anyone relying on the old redirect. Other Route redirects and redirect_slashes behavior are unaffected.

Verified by running pytest tests/test_routing.py.

[pctablet505]

Closing this for now — I want to rethink the approach before taking up review time here. Sorry for the noise.