fix: do not notify marked for deletion for deleted workspaces (#20937)

Closes https://github.com/coder/coder/issues/20913

I've ran the test without the fix, verified the test caught the issue,
then applied the fix, and confirmed the issue no longer happens.

---

🤖 PR was initially written by Claude Opus 4.5 Thinking using Claude Code
and then review by a human 👩

coderd/database/queries/workspaces.sql

@@ -846,6 +846,7 @@ SET
 WHERE
     template_id = @template_id
 	AND dormant_at IS NOT NULL
+	AND deleted = false
 	-- Prebuilt workspaces (identified by having the prebuilds system user as owner_id)
 	-- should not have their dormant or deleting at set, as these are handled by the
     -- prebuilds reconciliation loop.