feat: env leak detection

[florian-lefebvre]

Changes

• A new leakDetectionMiddleware is exported from astro/env/middleware. If a server var ends up on the client, an error will be thrown. Inspired by DMNO

Testing

Added a test

Docs

Changeset, I'll update the RFC after approval

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[ematipico]

It took me a while to understand that this middleware needs to inspect the Response. I would highlight this important detail, because putting this middleware on the front could allow users to change the secrets before the check.

[bluwy]

I wonder if we should make this astro:env/middleware instead?

[florian-lefebvre]

I think it makes things a bit more complicated. There's no codegen involved for the middleware code so I'm a bit reluctant to move it to a virtual import. We were also doing this for astro:env/setup and ended up moving it to astro/env/setup

[matthewp]

Let's set up a call and discuss this feature. Although it's very cool, I worry that the maintenance burden might be too high and not sure if we should take it on. But open to discussing for sure. We can set up a maintainers call to go over it.

[florian-lefebvre]

We need a proper RFC, this is a sensitive feature