Fix server islands for prerender-only sites

[bluwy]

Changes

fix #12803

Summary of issue:

1. Server islands work using endpoints.
2. It's added if Astro detects a page is being server-rendered, or if pages are configured to server-render by default (output: "server").
3. However, if the project happens to not set output (defaults to "static"), and all pages in src/pages are statically built (no export const prerender = false), then because of no2, the server island endpoints will not be added.
4. As a result, the build output is missing the /_server-islands/ handling and fails in the browser.

This PR:

• Fixes the issue by adding the server islands endpoint whenever it detects an adapter is being used. Should not be a problem for server-based adapters, but for static-based adapters, I believe it's still ok as they can specify in adapterFeaturs.buildOutput: 'static' and Astro will error that server islands won't work if they're used after building.

Testing

This one is a bit hard to write a test for, but I've updated some existing tests to match this new behaviour, and also manually ran the test in the adapters repo against this PR. It affects one Cloudflare test, but IIUC it's not breaking anything and should only require updating the code/test there to refine the output only. (This specific test)

Docs

n/a. bug fix.

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Latest commit: 8961688

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[codspeed-hq]

CodSpeed Performance Report

Merging #12982 will not alter performance

_{Comparing fix-server-island-static (8961688) with main (df90e6d)}

Summary

✅ 6 untouched benchmarks