#7226 - fixes NodeJS adapter for multiple set-cookie headers (and other header issues)

[alex-sherwin]

Fixes #7226

What it does

Utilizes the new standard WebAPI Fetch Headers.getSetCookie() function to safely handle multiple set-cookie headers when converting from a WebAPI Response to a NodeJS ServerResponse

Modifies the existing nodeMiddleware logic which used to first set AstroCookies on ServerResponse.setHeader(...) and then called ServerResponse.writeHead(status, Response.headers), which meant any that if the WebAPI Response had any set-cookie headers on it, they would replace anything from AstroCookies (they were not merged, so any AstroCookies would get lost during response writing, and, multiple set-cookie headers from the Response.headers were written incorrectly as a CSV).

The new logic delegates appending AstroCookies values onto the WebAPI Response Headers object, so that a new single unified function safely converts from the WebAPI Response Headers into a NodeJS compatible OutgoingHttpHeaders object utilizing the new standard Headers.getSetCookie() function provided by the existing undici WebAPI polyfills.

Plus extensive test coverage.

Notes

The Fetch WebAPI standard has been updated with a function Headers.getSetCookie() (see https://developer.mozilla.org/en-US/docs/Web/API/Headers/getSetCookie)

The undici library that @astrojs/webapi utilizes for WebAPI polyfills on NodeJS added support for Headers.getSetCookie() in 5.19.0 (see https://github.com/nodejs/undici/releases/tag/v5.19.0)

Since Astro is already using undici@^5.22.0, this means Astro code already had access to this method without having to change any dependencies.

I think that this is probably the best most standards compliant way to fix this problem, and other [unrelated] existing pieces of code that are using copy/pasted set-cookie heading parsing logic (such as https://github.com/withastro/astro/blob/main/packages/integrations/netlify/src/netlify-functions.ts#LL140C26-L140C26) might want to consider changing to using Headers.getSetCookie() where appropriate since going forward it's a standards compliant way of dealing with the set-cookie special case in the WebAPI Headers type.

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[natemoo-re]

This PR is incredibly thorough with great tests, thanks for contributing!

[alex-sherwin]

Thanks for merging!

I found the astro e2e/integration test harness stuff is really nicely done and was very easy to utilize for real-world integration/adapter test coverage