Trailing slash snafu when applying `adapter-node`'s `handler` to a dynamic base path

[HoldYourWaffle]

Describe the bug

Related: #595 and #7242.

I need to integrate a SvelteKit app into an existing Express.js server at a dynamic base path. Based on #595 (comment) I assume this is intended to be supported through the kit.paths.relative option, but there's a bit of a snafu with trailing slashes on the base path.

Standard static paths.base behavior (repro branch)

With a configuration like this:

const config = {
	kit: {
		adapter: adapterNode(),
		paths: {
			base: '/sveltekit',
			relative: true
		}
	}
}

...and integration into a custom server like this:

app.use(handler);

/sveltekit is always redirected to /sveltekit/, regardless of trailingSlash configuration. This makes sense, because the trailing slash is necessary to resolve relative paths starting with ./ correctly.
This seems to be handled by this snippet that eventually ends up in build/server/index.js:

kit/packages/kit/src/runtime/server/respond.js

Lines 298 to 301 in 0b60ea7

	// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
	// regardless of the `trailingSlash` route option
	if (url.pathname === base || url.pathname === base + '/') {
	trailing_slash = 'always';

Dynamic paths.base attempt (repro branch)

With a configuration like this:

const config = {
	kit: {
		adapter: adapterNode(),
		paths: {
			relative: true
		}
	}
};

...and integration like this:

app.use('/sveltekit', handler);

/sveltekit is no longer redirected to /sveltekit/, regardless of trailingSlash configuration. As expected this breaks basically everything with a relative path.

Reproduction

HoldYourWaffle/sveltekit-express-dynamic-basepath, specifically repro--static-base and repro--dynamic-base.

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 16.98 GB / 31.81 GB
  Binaries:
    Node: 22.14.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 10.6.2 - ~\AppData\Local\pnpm\pnpm.CMD
  Browsers:
    Edge: Chromium (133.0.3065.69)
    Internet Explorer: 11.0.19041.4355

Severity

serious, but I can work around it

Additional Information

After some digging I realized that app.use strips out the provided prefix path, which causes url.pathname to be / in both situations, therefore the aforementioned check can't tell the difference.
I assume #7242 would solve this issue, since there's no need to pass a prefix path to app.use if paths.base can be provided at runtime.

Workaround

In my case I can just implement the redirect myself outside the context of app.use:

app.all('/sveltekit', (req, res, next) => {
	if (!req.url.endsWith('/')) {
		res.redirect(308, '/sveltekit/');
	} else {
		next();
	}
});

app.use('/sveltekit', handler);