adapter: Fix: Don't ignore RETAIN HISTORY value for webhooks

[def-]

Broken by #31001. This is mostly adding a test, the fix is a one-liner, and then some code to have unreachable in case we add a new kind of table and forget this again.

Reproducer: bin/mzcompose --find retain-history down && bin/mzcompose --find retain-history run webhook-table

[github-actions]

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[mtabebe]

The change looks reasonable to me.

Out of curiosity, do we have any tests for the retain history given the real time component of it?

[def-]

Yes, test/retain-history, but it's been too flaky to enable, see the linked issue: https://github.com/MaterializeInc/database-issues/issues/7310