fix(html): don't treat a directory as the html file for a query url

[india3Eth]

fixes #23679

Since #23653 getHtmlFilename() strips the query, so transformIndexHtml('/?foo=bar', html) gets the root directory as filename. devHtmlHook only checked fs.existsSync(filename), which is true for a directory, so the URL was treated as a real HTML file and the inline <style> proxy module ended up with mod.file === '/', and ensureWatchedFile added / to the watcher.

It now requires filename to be a file (tryStatSync(filename)?.isFile()), so this case goes back to the virtual \0 proxy path it used before 8.3.3. I went with that over computing trailingSlash from the stripped path, since /?foo=bar + index.html would need extra handling to put index.html before the query.

Test in indexHtml.spec.ts: calls transformIndexHtml('/?foo=bar', …) with an inline <style> and asserts no directory gets watched. On main it watches /.

AI disclosure: I used an AI coding tool to help write this. I reviewed the change, checked the test fails without the fix, and ran the packages/vite unit tests (no new failures).

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