fix: better error when Sharp can't be resolved (ex: pnpm)

[Princesseuh]

Changes

Unfortunately, due to bundling, packaging, JavaScript, etc nonsense, on pnpm Sharp fails to be found. There's no proper way to fix this that wouldn't be: risky and/or unreliable. Trust me, I tried putting sharp in every single Vite config I found.

So, I did the next best thing: I upgraded the error message with every bit of info I could. I also added the passthrough / noop image service to the config so it's easier to use for SSR users who don't want to install Sharp into their projects. In the future, a more optimal solution for SSR would be a way to disable the endpoint, but that also requires either rethinking how we do images in Markdown, or at least a way to disable image processing in Markdown, it's a bit complex.

Fix #7966

Testing

Tested manually

Docs

/cc @withastro/maintainers-docs for feedback on the error message and description

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[ElianCodes]

Looks great!

[sarah11918]

Thanks for this great error, @Princesseuh ! I added some suggestions for details in case you wanted to make these even more super-explicit, as well as some links I know will be live in the new docs. (Some are now.) Your call!

My only concern was that (as you know from my trying to write these new docs!) "image processing" can risk sounding vague if people don't really get that "processed" doesn't include "simply display on your site". So this message makes me wonder in general maybe whether referring to "using astro:assets or not?" is an easier way for readers to understand that "processing" is more than just "agreeing to display your image on your page." 😄