Silently ignore adapters that don't export start()

[natemoo-re]

Changes

• Fixes a regression introduced by https://github.com/withastro/astro/pull/9776/files#diff-524e25c0bd5feda71ed7489963414a0c1f06da3caa327933574a72b85b668bddR264
• We got a report on Discord about Cloudflare failing to build in Astro 4.2.5
• Turns out that the obfuscated way this was originally written was very intentional because it bypasses some static analysis that bundlers try to do to warn against missing exports
• I added a comment to this effect so that this doesn't happen again

Testing

We unfortunately don't have automated adapter tests

Docs

N/A, bug fix only

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[lilnasy]

Thanks for reverting this!

[alexanderniebuhr]

lgtm