fix: allow using vite as a proxy for another vite server

[divdavem]

Description

The goal of this PR is to allow using vite as a proxy for another vite server, with HMR still working for the upstream server.

Additional context

Currently vite prevents any websocket connection with the vite-hmr websocket protocol from being forwarded to another server. I think it is not correct, vite should only handle itself the connection if it has the correct URL (in addition to the correct websocket protocol), and it should be able forward websocket connections to another server whatever the protocol it uses.

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.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.

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[patak-dev]

Would you explain your use case? Why is this feature needed in your setup?

[divdavem]

Would you explain your use case? Why is this feature needed in your setup?

The test case that I have included in this PR is basically a very simple reproduction of what we actually do.
We have an application (using vite) that includes multiple applications in iframes. During development, we use the proxy feature of vite in the outer application to include the inner applications at specific paths.
Some of the inner applications use vite, some others don't. One of the inner applications use angular, and the automatic refresh when changing a file works without issue (because the websocket used by the angular cli does not have vite-hmr in the Sec-WebSocket-Protocol header). The automatic refresh of inner applications that use vite do not work in the outer application that use vite. I opened this PR to fix this bug.

[patak-dev]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
astro	✅ success
histoire	✅ success
iles	✅ success
ladle	✅ success
laravel	✅ success
marko	✅ success
nuxt	❌ failure
nx	✅ success
previewjs	✅ success
qwik	✅ success
rakkas	✅ success
sveltekit	✅ success
unocss	✅ success
vite-plugin-pwa	✅ success
vite-plugin-ssr	✅ success
vite-plugin-react	✅ success
vite-plugin-react-pages	✅ success
vite-plugin-react-swc	✅ success
vite-plugin-svelte	✅ success
vite-plugin-vue	✅ success
vite-setup-catalogue	✅ success
vitepress	✅ success
vitest	✅ success

[sapphi-red]

I thought this is similar with vitejs/vite-plugin-vue#21. But actually this was different from that one.

I think this won't cause any issues for others. 👍

[divdavem]

@sapphi-red @patak-dev Thank you for reviewing and merging my PR!