fix: don't watch SPA fallback paths

[sapphi-red]

Description

This PR fixes the CI fail in vuejs/vitepress#3200.
https://discord.com/channels/804011606160703521/831456449632534538/1172931790554546368

The reason why it was failing with ELOOP: too many symbolic links encountered, stat '/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/gems/mechanize-2.9.1/mechanize-2.9.1' since beta.17 is because

• fix: add watch in fallback file load #14938 added ensureWatchedFile(watcher, id, root) after no plugin's load hook handled that id
• /home was passed to that because accessing /home goes through the plugin pipeline (

  vite/packages/vite/src/node/server/transformRequest.ts

  Lines 201 to 205 in b42a228

  	// if this is an html request and there is no load result, skip ahead to
  	// SPA fallback.
  	if (options.html && !id.endsWith('.html')) {
  	return null
  	}

  )
• ensureWatchedFile has fs.existsSync(file), but /home is true even though that is not a file

I moved the ensureWatchedFile after if (options.html && !id.endsWith('.html')). Instead, I can move some code from loadAndTransform to the plugin container, but I think it's safer to avoid that.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.